This can be due to wrong Workitem being passed , did you activate the MSS addon switch HCM_MSS_WDA_1?
for testing purposes you can check this application via Se80 and see if it works, I think it ll work in backend but while called throughPOWL It fails ? can you confirm?
in SWFVISU can you check the task for CATS
APPLICATION HRMSS_A_CATS_APPROVAL
DYNPARAM WL_ID=${item.externalid}&sap-wd-
configid=HRMSS_AC_CATS_APPROVAL
NAMESPACE SAP
If you also using UWL, you need to have this done
Please can you check if the note customisation 1600953 is implemented
inthe system and Note 1618269.
SYSTEM_ALIAS = SAP_ECC_HumanResources in SWFVISU for your task