Some domains, like eu.cc, that have already joined the PSL (Public Suffix List), when used on Vercel, still prompt:
This domain is linked to another Vercel account. To use it with this project, add a TXT record at
_vercel.eu.ccto verify ownership. You can remove the record after verification is complete.
The domain troubleshooting guide can help with most custom domain configuration issues. You might be able to use that guide to solve it before a human is available to help you. Then you can come back here and share the answer for bonus points.
https://vercel.com/docs/projects/domains/troubleshooting
You can also use v0 to narrow down the possibilities.
You still need to verify ownership even if it's added to the PSL I believe!
Can you add that TXT record?
Some old second-level domain suffixes in the PSL list do not require adding TXT records; only some newly added domains in the PSL need TXT records. As a user, I actually use third-level and fourth-level domains, and I am not the owner of the second-level domain, so I cannot add TXT records for the second-level domain.