@shivamtiwari2009 The support guide is here:
Netlify’s staff need to manually check the TXT record and remove the domain from other accounts.
It really should be self-service and a I made a feature request a few years ago regarding it:
https://answers.netlify.com/t/fix-another-site-is-already-using-this-domain-self-service/46443
The value that you chose might be ok, but following the procedure it should have been:
https://answers.netlify.com/t/stackknack-com-or-one-of-its-subdomains-is-already-managed-by-netlify-dns-on-another-team/124411