I'd ask you to post the workbook but I've just noticed that the buttons don't actually get disabled anyway, even when the code runs, so I would suggest switching to ActiveX buttons and using:

Private Sub Workbook_Open()
    With ThisWorkbook.Sheets("CommissionTest")
    
        If .Range("A18").Value = "" Then
            .CommissionMacro.Enabled = True
            .CommissionUpdate.Enabled = False
        Else
        
            .CommissionMacro.Enabled = False
            .CommissionUpdate.Enabled = True
               
            MsgBox "The Agencies tab needs to be updated." & vbNewLine & vbNewLine & "Please select the Update Agencies tab button"
            
        End If
    
    End With
End Sub