What you’ve done is incorrect:
Read the Support Guide again:
https://answers.netlify.com/t/support-guide-another-site-is-using-this-domain-verifying-domain-ownership-for-netlify-production-domain/13765
The name should be verified-for-netlify
The value should be:
https://answers.netlify.com/t/dns-issue-trying-to-add-a-domain-to-a-netlify-site-but-getting-error-another-site-is-already-using-this-domain/135897