Hi Skot,
I think I understand what is happening to you.
On the one hand, to avoid this error, you have to follow the steps of the corresponding states, I leave you a link so that you know them,
https://stockmanagementlabs.crunch.help/atum-purchase-orders-pro/po-status-logic
On the other hand, you can avoid this restriction if you want by changing the option in
ATUM Settings-> Purchase Orders->
I hope I have been helpful.
Best regards,
Juan Antonio.