This might be caused by two things:
1. Your PO date is in a Closed Period
2. You have entered a duplicate PO Number
If neither of these are the problem, try closing the PO screen and re-enter it fresh.
Having Issues? Try this
Permissions and Settings
To [action] you need permission to [name of permission]. To do this go to Admin > User > (Select User) > Change Permissions > [Settings Category] > [Name of Setting]
In order to [action], you need to [Description of Setting]. Click here to read how to set that up.