Only the user who created the sysmenus table in this database can modify the menu structure. That user first selected the Modify option of the User-Menu display while this database was current. To determine who that user is, query systables as follows:
SELECT owner FROM systables WHERE tabname LIKE 'sysmenu*'
If this action is not appropriate, ask that person to drop the sysmenus and sysmenuitems tables, and then have the person who should maintain the menus re-create them.