Domain 'abc.test.com' is being used by another Fastly account
Gatsby Hosting uses a Fastly account that is owned by Gatsby. Fastly doesn't allow a domain to be shared across Fastly accounts. If you see this error, it means that you have used that domain on another Fastly account. Sometimes this is because you have been integrated with Fastly in the past.
The fix is to go into the old Fastly account, find the service that is using that domain. Clone the service, delete the domain and then activate the service. Fastly won't allow the root domain "domain.com" to be shared so even if you added just a subdomain, "sub.domain.com", to your Fastly service, Fastly will use claim the root domain.