SAP Message AD237 - Only structures can be included in database tables

Diagnosis
A database table may not be included either directly or indirectly inanother database table.
This means that a structure that in turn includes a database table maynot be included either.

Procedure
You cannot include the table (or the structure containing a table).
You can:
Directly insert the fields.
Create a copy of the included DB table as structure and include it.