Union clauses in basis view & and extend & do not match

What causes this issue?

If a CDS view contains a UNION clause, identical UNION clauses must be
used in a corresponding CDS view extend. This means, type (UNION or
UNION ALL) and number of linked elements must be the same.

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.