Solution:

function loopMovement(movement, variation, range, a, b, c) {
  let arr = [];
  let projection = null;
  for (let i=0; i<range.length; i++) {
    if (range[i][a]===movement && range[i][b]===variation){
      projection = parseFloat(range[i][c]);
      arr.push(projection);
    }
  }
  var maxProjection = arr.reduce(function(a, b) {
    return Math.max(a, b);
}); 
  return maxProjection; 
}

and use the custom function like that:

=loopMovement("Squat","Competition",Sheet1!A$2:E$7,0,1,4)

Error:

The error message you are getting:

TypeError: Cannot read property '0' of undefined

has to do with the fact that the for loop iterates through empty rows. According to your screenshot, there are 6 rows excluding the header. However, for loop iterates until row 10.

Modifications:

I made the following changes to your script:

1. Instead of using the hardcopy value 10, you can use range.length to get the total number of rows of the given range.

2. movement and variation in the formula have to be passed as strings:

   =loopMovement("Squat","Competition",...)
   

3. Math.max() does not accept an array as an argument. For example, Math.max(1,5,2) returns 5, but Math.max([1,5,2]) returns NaN.

   Array.reduce() can be used to find the maximum element in a numeric array:

   var maxProjection = arr.reduce(function(a, b) {
   return Math.max(a, b);
   });
   

4. The expression range[i][c] returns a string with the desired value. However, you want to construct a numeric array, therefore you need to convert it to a float number (given that it might contain decimals) using parseFloat():

   parseFloat(range[i][c])