Hashnode uses Vercel to manage custom domains. This means if you add a custom domain on Hashnode, and later try to use the same domain on Vercel, you will see an error. For example, if you have added blog.foo.bar on Hashnode, you may not be able to use foo.bar on Vercel.
Please follow the steps below to fix this:
Remove the custom domain blog.foo.bar from Hashnode.
Add foo.bar on Vercel and follow instructions on Vercel Dashboard.
Come back to Hashnode, and add blog.foo.bar and follow the instructions in Hashnode Blog Dashboard.
You might have to set a TXT record in your DNS to prove ownership of the domain.
Hashnode will now be able to serve your custom domain as usual.