Seq.: Date &: Return operation for sequence not found.

What causes this issue?

The return operation specified for the sequence does not exist or has
been deleted. Therefore the task list cannot be edited or processed.

How to fix this error?

Check the task list and enter an existing operation as the return
operation for the sequence.
In order for the system to find a return operation from the standard
sequence that you have deleted and maintained later using the same
operation number, you should proceed as follows:

The system issues an error message and will not allow you to continue with this transaction until the error is resolved.

Enter an operation number which does not exist in place of the return
operation number
Save the task list
Rename the operation number you entered to the desired number

Error message extract from SAP system. Copyright SAP SE.