No unblocking request created; see long text

What causes this issue?

Unblocking requests are not created due to one of the following reasons:
Selection data not provided
Requested master data does not exist
Selected master data is not blocked

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?

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.