Purchase requisition & & blocked

What causes this issue?

Purchase requisition &V1&, item &V2&, is currently being processed by
another user or by you (in another session).

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?

If you are processing the requisition in another session, terminate
this processing and perform the function again.
Otherwise, try invoking the function again later.

Error message extract from SAP system. Copyright SAP SE.