Allowing macro to run while typing words into cells

Your cells do n't contain macros, you have code in the Worksheet object. What are you expecting, I can type a value into the specified rows & the calculation runs no problem.

This is a tidier way to write your code

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Column = 11 Then Exit Sub
    Select Case Target.Row
        Case 6, 7, 18, 4, 38, 42    '<<<< change cell to suit needs
            Application.EnableEvents = False
            Target.Value = Target.Value * 0.99
        Case Else: Exit Sub
    End Select
    Application.EnableEvents = True
End Sub

You can either use Data validation to only allow numbers or add an On Error statement

On Error Resume Next
'the code
On Error GoTo 0

Like this

Private Sub Worksheet_Change(ByVal Target As Range)
    If Target.Column = 11 Then Exit Sub
On Error Resume Next
    Select Case Target.Row
        Case 6, 7, 18, 4, 38, 42    '<<<< change cell to suit needs
            Application.EnableEvents = False
            Target.Value = Target.Value * 0.99
        Case Else: Exit Sub
    End Select
    Application.EnableEvents = True
On Error GoTo 0
End Sub