Table & cannot be accessed from primary table

What causes this issue?

An aggregate (matchcode object, view, lock object) contains several
tables which are linked together by foreign keys. The primary table is
linked to secondary tables by means of foreign key relationships.
Additional secondary tables can be added. The link from the primary
table to the secondary table can be transitive across several tables
which lie between these tables.
You specified a secondary table which cannot be accessed from the
primary table of the aggregate using a foreign key.

System Response

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

How to fix this error?

Check the foreign key relationships of the tables used in the
aggregate. Make sure that all the tables which lie on the path from the
primary table to the particular secondary table are contained in the
aggregate. If necessary, define the missing foreign keys in the table
maintenance screen of the corresponding table.
If this message was displayed when you activated a matchcode ID, not all
the tables on the path from the primary table to the table specified
in the message are marked as secondary tables in the matchcode ID.

Error message extract from SAP system. Copyright SAP SE.