If you only want a user to enter the purchase order, but not processing it, you can set it up by just check the “Enter Orders” checkbox under the Purchases section of the user privileges.
Both “Enter Orders” and “Process Orders” checkbox have to be checked if you need the user to perform both tasks.
However, you can’t have the user privilege of processing orders without having the privilege of entering orders, as Purchase Order need to be opened when processing take place.