Document &, item &, already being processed

What causes this issue?

You cannot open the chosen document at present because either it is
already being processed by another user or the system is currently
saving data from the processing of the document.

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?

Try again in a few minutes.

Error message extract from SAP system. Copyright SAP SE.