Select the primary table(s) in the piece list

What causes this issue?

You want to save an object definition in whose table list there is no
table marked as a primary table. At least one primary table is required
for an object definition.

System Response

The object definition is saved.

How to fix this error?

Go to the table list of the object and select the primary table(s) of
the object.

Error message extract from SAP system. Copyright SAP SE.