I cannot add <domain>.in.net

I tried to add kamontat.in.net domain in netlify dns but the error said, Subdomains are not supported as DNS zones

Yup, in.net looks like a domain to us, and our DNS hosting does not handle subdomains. While this may be a bug (did you buy “kamontat.in.net” from a registrar? If not, it’s not a bug…)

Unfortunately, you won’t be able to use our DNS hosting for that domain. You can still use our CDN though, following this guide: https://docs.netlify.com/domains-https/custom-domains/configure-external-dns/ with any other DNS host.

Yes, I bought this domain from Namecheap as screenshot shown.

Ref: https://domains.in.net/about-in-net/

Thanks! I’ve gotten a bug filed but do not expect the behavior to change in the nearterm, so my advice about using a different DNS host such as namecheap itself still stands.