Not sure whether you are using R12 or Cloud. But in R12, if Requisition is approved/cancelled/in process of approval/closed, then as per Oracle standard, it is not allowed to edit the requisition (or open the requisition).
Can you please tell me below question
1. are you using EBS or Fusion
2. What is the Requisition status at header level
3 check Document type - Purchase Requisition setup - is it mentioned as Public or Private.