The conversion has not yet taken place for the partner field

What causes this issue?

For the current release, the update of the partner field for tables
COSS and COEJ has changed in order to perform plan/actual comparisons
for settlement cost elements (type 21) in reporting.
These fields must be corrected for the existing database records before
planning is called up again.
For more information, refer to the release note
<DS:RELN.CO_RW_NEU30A_01>CO Reports: Displaying Clearing Entries With
Orders</>.

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?

Start the report <ZH>RKACUM26</>.
<DS:TRAN.SE38>Proceed</>
After the report has run correctly, you can call up planning again and
continue.

Error message extract from SAP system. Copyright SAP SE.