The changes are posted under the numbers & and &

What causes this issue?

The planning changes have been made correctly. After the update, they
can be used for further evaluations. The changes can later be
reproduced using the plan line items which were posted under the above
numbers.
Two plan documents were created because you made planning changes both
for primary cost elements (1st document number) and for order cost
elements (2nd document number).

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?

Error message extract from SAP system. Copyright SAP SE.