Hey, @rosejay .
Tira here to help.
Thank you for your question and for these details. This is helpful!
I see that you’re looking to add an app to your store so that you can ship with Royal Mail. Is that right? If it is, can you confirm that it’s this app from our App Store: Royal Mail Shipping Extension by Developify?
I just wanted to provide some insight. If you’re looking to use a third-party shipping carrier on your store, then you’re receiving this error message because you will need to have carrier calculated shipping added to your store so that you can use the app on your store. Carrier-calculated shipping will allow you to display your own negotiated rates from your shipping carrier. This feature is included with our Advanced plan and annual plans or can be added onto your account for a monthly fee. You can learn more about third-party carrier calculated shipping in our help docs.
If you don’t want to upgrade your subscription plan, you can contact our support team to help you add this feature to your store for a monthly fee.
We will need to access your account in order to resolve this issue. While we’re not able to provide account-specific support via the Shopify Community at this time, we’d be happy to continue assisting you through live chat, email, or callback. Please visit https://bit.ly/3cJkx8V and log in to your account to create a support request.
Since this is the next step I’ve marked this reply as the solution for the benefit of others who may discover this topic. Thanks!