This might be a data issue with the PO you are trying to release.
You can debug as below.
Goto Class CL_MM_PUR_PO_APPR_ACTION_BADI, method SET_DECISION_RELEASE. Search for "e017(mm_pur_po_apv_msg)". Put breakpoints at all the three places and trigger release from My Inbox.
While trying to release the workflow from the Fiori Inbox, check transaction SU53 in your backend system to see if the user is missing any authorizations.
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