Extension & is not assigned to any basic type

What causes this issue?

Extension &v1& is to be used as a model for a new extension.
But is has no basic type assigned.

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?

Only extensions assigned to a basic type can be used as models.
Alternatively, you can also state a basic type explicitly.

Error message extract from SAP system. Copyright SAP SE.