VBA-related question: Compiler Error Variable Not Defined [duplicate]

I am just starting to learn VBA. I am using the following code but am getting an error when I try to run Sub test:

Compile Error: Variable not defined

Can you help me figure out what is wrong?

Private Sub CommandButton1_Click()
        Dim dblPrincipalAmt As Double
        Dim dblInterestRate As Double
        Dim intYears As Integer
        Dim dblInterestAmt As Double
        
        If Not IsNumeric(txtPrincipalAmt) Then
            MsgBox "INVALID PRINCIPAL AMOUNT"
            Exit Sub
        End If
        
        If Not IsNumeric(txtInterestRate) Then
            MsgBox "INVALID INTEREST RATE"
            Exit Sub
        End If
        
        If Not IsNumeric(txtYears) Then
            MsgBox "INVALID YEAR"
            Exit Sub
        End If
        
        Let dblPrincipalAmt = txtPrincipalAmt
        Let dblInterestRate = txtInterestRate
        Let intYears = txtYears
        
        Let dblInterestAmt = dblPrincipalAmt * dblInterestRate * intYears
        Let TBIA = dblInterestAmt
    
    
End Sub

I am trying to find the error so that I can fix it right away.