A data base index is missing for using the ISR number

What causes this issue?

If the vendor is to be determined using the ISR subscriber number, a
corresponding data base index must be created. That is not the case in
this installation.

System Response

You cannot activate the required option.

How to fix this error?

Ask the data base administrator to create an index for table <ZH>LFA1</>
which uses field <ZH>ESRNR</> as the next field after the client. The
index must be described and created with SAP transactions so that the
information on the existence of the index is stored in the SAP System.

Error message extract from SAP system. Copyright SAP SE.