Reverse all overlapping postings first

What causes this issue?

There are already postings for the new fixed period. The fixed period
end of the old contract was probably changed.

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 whether the postings are correct and reverse them if necessary.

Error message extract from SAP system. Copyright SAP SE.