Semantic key element &1 is unknown or hidden.

What causes this issue?

The element used in the semantic key annotation is invalid. Only
elements that are exposed in the CDS view and that are not annotated as
'@Consumption.hidden' can be used as semantic 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?

Use a valid element at the semantic key annotation 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.