Subrequest & does not exist in system

What causes this issue?

The system needs to access a transport request, however, it does not
recognize this transport request.

System Response

The function terminates.

How to fix this error?

Check whether the step "Create Transport Requests" has started. If not,
repeat the export step.
Check whether there were errors in the step "Create Transport
Requests". Any errors could be caused by system settings or
authorization problems.
Use the Transport Organizer (Transaction SE09) to check whether the
specified transport request really does exist in this system.

Procedure for System Administrators

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Error message extract from SAP system. Copyright SAP SE.