Did you have a support ticket number you could give me? I'd be happy to look into this for you!
Sorry to see you're having trouble here. Errors like this can pop up if there is some kind of disconnect between your Kit account and Facebook. You may have accidentally disabled the account for a moment and they reconnected, or Facebook my have some hold on the account from their end. I've reached out to the email address linked to your Forums account so I can confirm your Shopify store, after that I can have a chat with our Kit support team so they can have a look into your account! I'm sure we can get to the bottom of this for you.
Also, if you have an error with Kit or a question for the support team, you can reach out to email@example.com for some direct help :)
Sorry to hear you are having some trouble here. I'd recommend reaching out to firstname.lastname@example.org to get in touch with our dedicated Kit support team. They will be able to take al look at the back-end of your Kit account and will be able to troubleshoot in greater detail for you. I'm sure they can resolve this for you!
Let me know how you get on,