CHANGE CELL BASED ON TWO OTHER CELLS' VALUES AS COORDENATES

Hello,
I am new at this website and in vba programming and I am looking for some assistance with my vba code. I'm trying to right a code that makes a change and then try to applies it on other tab by taking the value of one cell that returns the number of the row and the value of the next cell that returns the number of the column and select that cell and applies the change.
the error is at Cells(a, b).Select
thanks in advance for your time and help!

If Target.Row < 500 And Target.Column = 6 Then 
For x = 8 To 500 
If Cells(x, 6).Value = "C" Then 
a = Cells(x, 12).Value 
b = Cells(x, 13).Value 
Sheets("13").Select 
ActiveSheet.Protect "1", DrawingObjects:=True, Contents:=True, Scenarios:=True, AllowFiltering:=True 
ActiveSheet.Unprotect "1" 
Cells(a, b).Select 
ActiveCell.ClearContents 
ActiveCell.FormulaR1C1 = "C" 
ActiveSheet.Protect "1", DrawingObjects:=True, Contents:=True, Scenarios:=True, AllowFiltering:=True 
Sheets("VERSO").Select 
End If 
Next x 
End If

Diego Simões