Empty required screen field doesn't give error

Thanks all for your input.

You (all) ask me to program extra code (in PAI or UE).

But if you define the field as required SAP should handle this by std.

And in fact it does give an error message. But I need to be in the correct screen.

Why does SAP at Save not check all fields in all tabs (screens)?

(I expected this to be std in SAP; like eg in ME21N/VA01 (create pur./sls order))