Problem
Finally, you are set up your project and hosted it on Vercel, But when you want to add a custom domain this error happens:
Error: The domain is already owned by another Vercel account.
I will talk about my case, then, explain other cases.
I own essaidi.me domain, and in the past, I added a lot of sub-domains for multiple web apps, and everything is working fine, But, when I want to add essaidi.me, I was surprised that domain is already in another account.
I've reached the customer support of Vercel, and they informed me that my domain (actually subdomain) is associated with Hashnode, which is a platform of blogging for developers because I already made a sub-domain called blog.essaidi.me.
But wait! Hashnode is another thing right? well actually according to the support, Hashnode runs their system on top of Vercel, and the domain is associated with the Hashnode Vercel Account.
For other people who have not related to Hashnode and want to add just a sub-domain, and they already added a domain on their Vercel account, Vercel said that they will be rolling out an update to officially support this use case.
Solution
Quick Solution
If you have already a sub-domain or domain in the Hashnode, contact them and tell them to remove your domain so you can use it on Vercel. But, that means that your subdomain on Hashnode will be lost.
Best Solution
If you don't want to remove your sub-domain from Hashnode, or you already have a domain in Vercel. the only solution is the wait for the upcoming updates by Vercel.
According to their support, the update will allow domains to be shared between different Vercel accounts in the near future.