Diagnosis
You have no authorization for the authorization object 'S_DEVELOP',which would allow you to change the screen concerned.
There is also a second possibility:
You want to edit a modified SAP screen in a customer system after anupgrate (SAP transport). The screen remains locked for editing untilthe new SAP version and the customer version have been aligned(Transaction SPAU).

System Response
Authorizations are assigned to users in profiles. Profiles are storedin the user master record.

Procedure
Comsult your system administrator.