Question:
Entries of invalid values in area A1:B6 are to be prevented, in case of an error a respective message box to be displayed.
Answer:
Insert the following code in This Workbook module.
Private Sub Worksheet_Change(ByVal Target As Range)
Dim var As Variant
If IsEmpty(Target) Then Exit Sub
If Intersect(Target, Range("Calculating")) _
Is Nothing Then Exit Sub
var = Application.Sum(Range("Calculating"))
If IsError(var) Then
Beep
MsgBox "Please enter the correct value!"
Target.Value = ""
End If
End Sub