ACME v1 account registrations seem to be disabled only on staging (acme-staging.api.letsencrypt.org) as part of a brown-out period:
I'm not sure why the Azure ACME client would be using the staging environment for live certificates.
I'm not sure whether GitHub - sjkp/letsencrypt-siteextension: Azure Web App Site Extension for easy installation and configuration of Let's Encrypt issued SSL certifcates for custom domain names. (which I guess you are using) supports ACMEv2 or not. It seems to - maybe you need to upgrade the extension or the configuration you are using somehow?