When you are setting up a business flow for purchase requisitions you should check what permissions someone needs to place requisitions on behalf of other people. For example perhaps an administrator for a department might have permissions to put in a requisition on behalf of the member of the team. Or the purchasing staff have permission to place it on behalf of employees. Sometimes this also varies by company. You can set these up in AX under the purchase requisition permissions.
Here is a quick overview.
AX2012 R3 CU11