if ModuleCode is a numeric field, change the DCount line to read
If DCount("[ModuleCode]", "tblStaffModules", "[ModuleCode] = " & Me.CboModule) = 0
If ModuleCode is a text/alpha field, use this:
If DCount("[ModuleCode]", "tblStaffModules", "[ModuleCode] = " & "'" & Me.CboModule & "'") = 0