This is the suggested code:

Sub ErrorToZero()
   X = Right(ActiveCell.Formula,Len(ActiveCell.Formula)-1)
   ActiveCell.Formula = "=IF(ISERROR(" & X & "),0," & X & ")"
End Sub

I'd use:

Option Explicit
Sub ErrorToZero2()
   Dim myRng As Range
   Dim myCell As Range
   Dim myStr As String
   
   Set myRng = Nothing
   On Error Resume Next
   Set myRng = Intersect(Selection, _
                     Selection.Cells.SpecialCells(xlCellTypeFormulas))
   On Error GoTo 0
   
   If myRng Is Nothing Then
       MsgBox "No formulas in selection!"
       Exit Sub
   End If
   
   For Each myCell In myRng.Cells
       myStr = Mid(myCell.Formula, 2)
       myCell.Formula = "=If(iserror(" & myStr & "),0," & myStr & ")"
   Next myCell
   
End Sub