Index &1 of table ECMCC was activated under warnings

What causes this issue?

The unique secondary index &V1& must be generated for table ECMCC. It
was activated with warnings.

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?

Examine the index's activation log.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.