If cell is empty and macro is pressed display error message.

You can try this... it should show a message box with the error if EITHER cell is blank. But since we don't have your code, it's tough to be sure...

Sub Blanks_OR()
With Worksheets("Sheet1") ' Change to your sheet name here
If IsEmpty(.Range("D2")) Or IsEmpty(.Range("A12")) Then
MsgBox "Please fill in the invoice."
Else
' Put your macro code here
End If
End With
End Sub