The domain [domain] was added by another HubSpot customer. If you believe that another customer account is using this domain in error, please contact support.
This error might appear when you try to add a subdomain to HubSpot that's already been added to another HubSpot account. For example, if you have a trial account that's different than your paid account, you would see this error if the same domain was added to both places.
To resolve this error, please remove the related domain from Domain Manager of the account that you're not using with the domain. If you're unable to access that account or you believe another customer account is using the domain in error, you can reach out to HubSpot Support here.