Object definition (&1) tables have no cust. delivery class

What causes this issue?

The piece list of object &V1& (see transaction SOBJ) contains at least
one table/view with an inappropriate delivery class for customizing
data, or which is a system object (e.g. table T000 , TADIR, CVERS,
....).
Tables/views which are used in object &V1&, and are customizing (and BC
Set) relevant, must not be system objects, and must have one of the
following delivery classes (transaction SE11):
'C': customizing table, maintained by customers, not an SAP import
'G': customizing table, protected against SAP UPD, only INS allowed
'E': control table, SAP and customers have their own key areas
SPACE: is also accepted as a customizing delivery class
<ZK>System objects:</> System objects are tables which are either in the
ABAP Dictionary, or internal SAP system tables. They cannot be
transported, so they cannot be used in BC Sets.

System Response

You can put the object in BC Sets, but the tables/views which have a
delivery class which is not allowed in BC Sets, or are system objects,
cannot be used in BC Sets, so they are not offered for selection in the
processing transaction.

System Response

The object, but not the tables/views, can be put in BC Sets.

How to fix this error?

Check the delivery class of all tables in the SOBJ piece list. You can
contact the SAP developer responsible.
If all subobjects have a customizing delivery class, and this message
still appears, at least one table is a system object, and the object
&V1& cannot be put in a BC Set.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.