Can't insert data into SQL Server table in a loop using c#

What you really should do is:

• create the list of parameter objects once, before the loop
• during the loop, only set their values

Something like this:

string InsertData = "INSERT INTO AUStagAPITestData ([TestSuite], [TestCase],[Status], [Info], [Time], [IsArchived], [DateTime]) VALUES (@TestSuite, @TestCase, @Status, @Info, @Time, @IsArchived, @DateTime)";

// put your connection and command into *USING* blocks to properly dispose of them
using (SqlConnection sqlconn = new SqlConnection(sqlconnectionstring))
using (SqlCommand Insertcmd = new SqlCommand(InsertData, sqlconn))
{
    // create the parameters **ONCE** and define their datatypes
    // I have only *guessed* what the datatypes could be - adapt as needed
    Insertcmd.Parameters.Add("@TestSuite", SqlDbType.VarChar, 50);
    Insertcmd.Parameters.Add("@TestCase", SqlDbType.VarChar, 50);
    Insertcmd.Parameters.Add("@Status", SqlDbType.VarChar, 50);
    Insertcmd.Parameters.Add("@Info", SqlDbType.VarChar, 50);
    Insertcmd.Parameters.Add("@Time", SqlDbType.Time);
    Insertcmd.Parameters.Add("@IsArchived", SqlDbType.Boolean);
    Insertcmd.Parameters.Add("@DateTime", SqlDbType.DateTime);

    sqlconn.Open();

    // now loop over the data and set the parameter values
    for (int j = 1; j < TDData.Length; j +=5)
    {
        string TestSuite = TDData[j];
        string TestCase = TDData[j+1];
        string Status = TDData[j + 2];
        string Info = TDData[j + 3];
        string Time = TDData[j + 4];

        Insertcmd.Parameters["@TestSuite"].Value = TestSuite;
        Insertcmd.Parameters["@TestCase"].Value = TestCase;
        Insertcmd.Parameters["@Status"].Value = Status;
        Insertcmd.Parameters["@Info"].Value = Info;
        Insertcmd.Parameters["@Time"].Value = Time;
        Insertcmd.Parameters["@IsArchived"].Value = true;
        Insertcmd.Parameters["@DateTime"].Value = DateTime.Now;

        // execute the query in the loop
        Insertcmd.ExecuteNonQuery();
    }   

    sqlconn.Close();
}

It's complaining you have already added:

Insertcmd.Parameters.AddWithValue("@TestSuite

The fix is to instantiate a new SqlCommand each iteration:

for (int j = 1; j < TDData.Length; j +=5) 
{
sqlconn.Open(); 
SqlCommand Insertcmd = new SqlCommand(InsertData, sqlconn); 
string TestSuite= TDData[j];

try to move sqlconn.Open() & sqlconn.Close() out of the for loop and renew sqlcommand object.

sqlconn.Open();
for (int j = 1; j < TDData.Length; j += 5)
{
    SqlCommand Insertcmd = new SqlCommand(InsertData, sqlconn);
    string TestSuite = TDData[j];
    ...
}
sqlconn.Close();

Check this example based on your code:

string connectionString, queryInsert;
string[] arrayData = new string[10];

connectionString = ConfigurationManager.ConnectionStrings["DbConn"].ConnectionString;
queryInsert = @"
    INSERT INTO AUStagAPITestData
    (
        [TestSuite], [TestCase], [Status], [Info], [Time], [IsArchived], [DateTime]
    )
    VALUES (
        @TestSuite, @TestCase, @Status, @Info, @Time, @IsArchived, @DateTime
    )
";

using (SqlConnection connection = new SqlConnection(connectionString))
using (SqlCommand command = new SqlCommand(queryInsert, connection))
{
    string testSuite, testCase, status, info, time;

    connection.Open();

    for (int j = 1; j < arrayData.Length; j += 5)
    {
        testSuite = arrayData[j];
        testCase = arrayData[j + 1];
        status = arrayData[j + 2];
        info = arrayData[j + 3];
        time = arrayData[j + 4];

        command.Parameters.AddWithValue("@TestSuite", testSuite);
        command.Parameters.AddWithValue("@TestCase", testCase);
        command.Parameters.AddWithValue("@Status", status);
        command.Parameters.AddWithValue("@Info", info);
        command.Parameters.AddWithValue("@Time", time);
        command.Parameters.AddWithValue("@IsArchived", "1");
        command.Parameters.AddWithValue("@DateTime", DateTime.Now);

        command.ExecuteNonQuery();

        // To Clear parameters
        command.Parameters.Clear();
    }
    // no need to close a disposed object since dispose will call close 
}

You can use Insertcmd.Parameters.Clear() inside your for loop.

As I wrote in the comments, I would use a stored procedure with a table valued parameter instead of inserting the records one by one. This has the advantage of only making one round trip from your application code to your database.
However, it also has a disadvantage - if one row fails for any reason (say, violating a constraint), the entire insert will fail.

Having said that, in order to use a table valued parameter you should first create a user defined table type. Note that I'm guessing your columns data types here, you might need to change them:

CREATE TYPE dbo.tt_TestData (
    [TestSuite] int, -- I'm guessing foreign keys
    [TestCase] int,
    [Status] int, 
    [Info] nvarchar(255), 
    [Time] time, 
    [IsArchived] bit, 
    [DateTime] datetime
);
GO

After you've done that, you can create your stored procedure:

CREATE PROCEDURE stp_AUStagAPITestData_Insert
(
    @Data dbo.tt_TestData READONLY -- Note: Readonly is a must!
)
AS

INSERT INTO AUStagAPITestData (
    [TestSuite], 
    [TestCase],
    [Status], 
    [Info], 
    [Time], 
    [IsArchived], 
    [DateTime]
)
SELECT 
    [TestSuite], 
    [TestCase],
    [Status], 
    [Info], 
    [Time], 
    [IsArchived], 
    [DateTime]
FROM @Data;
GO

Now, to execute this stored procedure using ADO.Net, you will need to create a data table for your data, and send it as a paramameter of type SqlDbType.Structured to the stored procedure:

using (var sqlconn = new SqlConnection(sqlconnectionstring))
{
    using (var Insertcmd = new SqlCommand("stp_AUStagAPITestData_Insert", sqlconn))
    {
        //  Create the data table
        using (var dt = new DataTable())
        {
            dt.Columns.Add("TestSuite", typeof(int));
            dt.Columns.Add("TestCase", typeof(int));
            dt.Columns.Add("Status", typeof(int));
            dt.Columns.Add("Info", typeof(string));
            dt.Columns.Add("Time", typeof(DateTime));
            dt.Columns.Add("IsArchived", typeof(bool));
            dt.Columns.Add("DateTime", typeof(DateTime));

            // Populate the data table from the TDData string array
            for (int j = 1; j < TDData.Length; j += 5)
            {
                dt.Rows.Add
                (
                    TDData[j],      // TestSuite
                    TDData[j + 1],  // TestCase
                    TDData[j + 2],  // Status
                    TDData[j + 3],  // Info
                    TDData[j + 4],  // Time
                    true,           // IsArchived
                    DateTime.Now    // DateTime
                );
            }

            Insertcmd.CommandType = CommandType.StoredProcedure;
            Insertcmd.Parameters.Add("@Data", SqlDbType.Structured).Value = dt;

            try
            {
                sqlconn.Open();
                Insertcmd.ExecuteNonQuery();
            }
            catch (Exception e)
            {
                // Exception handling code goes here...
            }
        }
    }
}