Permissions for approving Purchase Orders

Hi, trying to find permissions that would give access only for approvinng Purchase Orders. Tried useing the Vanilla role but no luck and much frustration trying to make one manually. how can I find what permission is lacking ? only message is :

Stopped (error): X++ Exception: Work item could not be created. Insufficient rights for user axtest5.
at SysWorkflowWorkItem-create
SysWorkflowWorkItem-createWorkItems
SysWorkflow-save
SysWorkflowQueue-resume

Any help would be so appreciated. thanks!