This can happen in a few cases:
1. The user has no supervisor in HR record
2. One of the AME approval group may be dynamic & the query is based on user. That query might be failing to generate the list for the group.
3. One of the member of his supervisor hierarchy might have a corrupted HR record or has left.
Please verify these possibilities and provide more details.
Oracle is not tough. Please reply to the following questions, you will get your answer.
a) Pick up the Requisition header Id from the Backend using the query, select header_id from po_requisition_headers all where segment1 = "REQUISITION NUMBER"
b) Navigate to Test Workbench in AME responsibility
c) Click on Run Real transaction test, enter the transaction ID as the Header ID you got from the above query
d) Run the test case, observe which rules are being picked up for this requisition.
e) Analyze the approval groups/ approval authority setup based on the rules being invoked.
If no rules are applicable, then create one as per the business requirement.