Diagnosis
You probably want to maintain a plant which you have already created asa customer in SD. You cannot extend this using 'Create' transactions.

Procedure
Check that your entries are correct.
If necessary, maintain your data using customer transactions.
If you want to change a customer into a plant, contact your systemadministrator. The system administrator can, if required, create anentry in table T001W which references the customer concerned. Thecustomer is thus linked with plant master data processing and can beprocessed using Change transactions.