This is a known issue that is affecting everyone using Click & Drop with Shopify, see this thread https://community.shopify.com/c/Shopify-Discussion/Royal-Mail-Click-and-Drop-stopped-marking-orders-...
So uninstalling/reinstalling etc. isn't going to help i'm afraid.
Don here from Shopify.
As @JonWright mentioned above, this is a known issue affecting that Royal Mail service as integrated with Shopify.
We're aware of this and working to resolve what we can from our side, though our only advice at the moment is to reach out to Royal Mail directly to see if there's anything they would suggest.
Otherwise, it would just be necessary to wait while this is being worked on.
All the best!
This is an accepted solution.
Keeping threads together https://community.shopify.com/c/Shopify-Discussion/Roal-Mail-Click-and-Drop-Down/m-p/679160#M160181
I've just tried it and it's back up & working for me as well.