Not currently possible to release against contract & (being processed)

What causes this issue?

You wish to create a purchase order referencing contract &v1&. This is
not currently possible because either another user is in the process of
creating a release order against this contract or the contract itself
is being processed. If another user is processing the contract, the
latter is blocked. It is possible that a system error occurred during
blocking.

System Response

The item cannot be created without errors.

How to fix this error?

You have the following options:
Enter the PO without the item, put the PO "on hold", and then process
it later (using the "check" function)
Invoke the "check" function at a later point in time while processing
this PO
The system will then make another attempt to access this contract.

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.