I have done all these things, and I actually verified the domain successfully before in December, but for some reason it is not working for me now either.In another thread (https://forums.developer.apple.com/thread/122124) there are other things to check (redirects and SSL ciphers and what-not) but none of that is under our control with firebase hosting, and either way I've verified by direct request inspection (HTTP response codes and headers etc) that it's a plain 200 response with the file as expected from the firebase hosting domain[Edit / update]: I have requested support from the firebase team. I'm a react-native-firebase maintainer so hopefully they help me and we can figure it out :-)[Second edit / update]: it occurred to me that I had successfully verified a different iOS app but it was this same firebase backend (so the same domain and handler URLs) and sure enough I still had that service ID verified / connected, which is not valid because you can only have a domain linked to one at a time. So I deleted those old services with the verified domains and re-tried, and it still failed. At this point it might be transient - failing only because I deleted the associates so recently - or it might be persistent. But there is at least one valid reason why my validation was failing. Hopefully this resolves at least and I am able to re-verify the domain under the new iOS app as I was able to do in DecemberFinal update: I successfully verified the domain. You need to make sure it is not already associated somewhere else (remove it if needed). Configure the domain as if you were going to verify it then make sure it's saved. Then go back in, verify the domain is correct and download / re-push the file to public hosting, and verify. It should work. This is with the default "yourapp".firebaseapp.com domains and auth handlers.

Did you ever resolve this?I have 2 apple developer accounts, and 3 domains that I verified as sign-in-with-apple services on one account and am attempting to move to the other accountI deleted all the services (which according to apple docs should remove the association) from one accountI successfully verified 2 of the domains with the receiving apple developer account, but got this error for the third domain I attempted to verifyI audited the original developer account and it no longer has the services so that should free the domain up for verification, as shown by success with the other 2 domains.I can verify that if I re-add the domain to the original account it shows up as verified already.Then if I re-delete the service, I can verify it in the second account.So perhaps you just need to try again, and Apple's "remove domain association" feature in their back-end code has an error of some kind.Very strange.