Purchase Document running into "No Approver Found"

 Hi we are on EBS 11i, we are facing some issues during Approvals of Purchasing Document, Whenever Buyer submits the PO for approval it runs into "No approver found", the same document needs to submit twice then the PO status change to "Inprocess" we are not able to figure out why this is happening have gone through various note id's followed various checks. Can anybody help me in solving this issue or get me to the exact cause of this issue.

Thanks!