Welcome to the Community, and thanks for including the screenshot in your post!
It appears as though you do have the Cash On Delivery payment method set up correctly. However, the alert that you are seeing at the checkout is most likely due to your store being a Development store.
Do you recall if it is? You can check it by going to ‘Settings > Plans and permissions (or Account)’ from your Shopify admin. A Development store will be identified like this:
Development stores are set up to allow for an unlimited number of test orders. However, since a manual payment method such as Cash on Demand is not actually processed through any type of payment gateway, there is no way to have it in ‘test mode’; it’s considered a real transaction. You can read more about development stores and their features and limitations here.
Moving forward, to be able to process real transactions, you’ll need to:
Hope this helps! Let me know if you have any further questions.