Why do I see an upgrade message when I click Create order?

Creating orders from the admin is part of the Backend Order plugin. The Create order button is always on Orders → Manage Orders, but the order screen only opens when your store has this plugin. Without it you see an upgrade card instead. Check Plugins, or the add-ons included in your plan.

If you do have the plugin but Place Order fails with a message that the order can't be processed right now, your plan's order limit has probably been reached. Upgrade your plan to keep placing orders.