Reversal for document with key &1 not possible, process first ECS item &2

What causes this issue?

Posting of document with key <(>&<)>1 is pending as an ECS Item <(>&<)>2
is
created which is not completed yet.

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

System Response

Reversal for document <(>&<)>1 rejected

How to fix this error?

You can either reverse the corresponding ECS item <(>&<)>2 with
ACC_ECS_REVERSE or if postings are required correct ECS item to
create the missing posting and trigger the reversal of the ECS item
afterwards which will create a cancellation document.

Procedure for System Administrators

Error message extract from SAP system. Copyright SAP SE.