runtime error '13' type mismatch

Try this:

Dim lngrows As Long
lngrows = Sheets("sheet3").UsedRange.rows.Count
For i = 2 To lngrows
    If IsError(Cells(i, 44).value) Then
        Cells(i, 44).ClearContents
    End If
Next i

Note: I renamed your rows variable as rows is already an Excel property and I changed the data type to Long rather than integer since there are more rows in a sheet than an integer can cope with.