Do not use the virtual element &1 in the association definition.

What causes this issue?

In the consumption CDS view, a virtual element is used in an association
definition. This is not allowed.

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?

Remove the virtual element from the association definition in the CDS
view.

Procedure for System Administrators

Contact the corresponding developer of the CDS view to correct the
error.

Error message extract from SAP system. Copyright SAP SE.