You have a sub-object that can't be API enabled and gives the ODF-0248 error. It needs to be API enabled for a process and it is a 3rd level sub-object under another custom sub-object which is under Project object. The parent object(s) are API enabled. When creating a new sub-object under 2nd level sub-object you aren't able to enable it either, however if you create a new sub-object under Project, then create a new sub-object under that, then you can enable it for API.
Clarity 16.3.3
ALLOW_CUSTOM_SUB_OBJ_APIALIAS field on odf_objects table for the subobject was not enabled.
Something might have gone wrong when initially the object was API enabled.
You can run below update statement followed by services restart or flush caches.
UPDATE odf_objects SET allow_custom_sub_obj_apialias = 1 WHERE code = 'your_subobject_code';
COMMIT;