Which value did you pass in MEMORY, HOLD/PARK_COMPLETE/UNCOMPLETE fields, and which error did you get in RETURN parameter?
Hint: If your system is not up-to-date, check OSS note 1608543 - BAPI_PR_CHANGE: cannot save parked requisition, you could also set a break-point in method process_req of class lcl_bapi_header for fields as l_uncomplete and l_post.
Regards,
Raymond