Diagnosis
When maintaining matchcode sub-keys for a screen field, you failed tospecify either the field ID or the field name of the matchcode sub-key.

System Response
To assign a matchcode sub-key to a screen field, you must specify boththe field ID and also a field name. Therefore the system rejects yourinput.

Procedure
With matchcode selections, matchcode sub-keys allow you not only toassign a selected value to the screen field where you make thematchcode entry, but also to fill other screen fields as a result ofthe selection.
When you define a matchcode object in the ABAP/4 Dictionary, you canassign a field ID to a matchcode field. If a matchcode record isselected, the value of the field is attached to the field ID (as withGET/SET parameters).
If there is a matchcode sub-key for this field ID, the system placesthe value attached to the field ID in the screen field whose field nameis specified as the relevant sub-key field name.
The functionality requires you to specify both a field ID and a sub-keyfield name for a matchcode sub-key.