Assign/deassign not possible due to existing call-offs for document &

What causes this issue?

It is not possible to assign or 'deassign' the document if any call-offs
exist for 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?

Check if the entered document number is correct.

Error message extract from SAP system. Copyright SAP SE.