This is an accepted solution.
Elias here with Shopify Support. That's a great question!
Just to clarify, in order to process orders or allow customers to place orders, your store needs to be on one of our eligible plans that provides an activated checkout. Nonetheless, not to worry, there isn't anything wrong with your theme. Though there aren't any processing fees with Manual payment methods, such as Cash on Delivery (COD), simply use any one of our paid plans to have an activated checkout. However, it's important to note that this does exclude the Full Paused plan and trial stores.
I hope this helps! If you have any other questions on this topic, don't hesitate to let me know!