& requisition(s) currently being processed by other users

What causes this issue?

You have attempted to change a purchase requisition from within list
processing. However, in the meantime this requisition is being
processed by another user.

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?

Processing is not currently possible. Try again later.

Error message extract from SAP system. Copyright SAP SE.