I can not add a new site

When i add site xn–bq8h52g.to (this is emoji domain) i have error

We were unable to identify xn–bq8h52g.to as a registered domain. Please ensure you are providing the root domain and not any subdomains (e.g., example.com, not subdomain.example.com) (Code: 1099)

At the registration i change NS servers to cloudflare
dig xn–bq8h52g.to NS showed me NS servers cloudfalre

Please tell me, what do i do wrong?

I presume you did add xn--bq8h52g.to and not xn–bq8h52g.to and Discourse merged the two hyphens into one, right?

There seems to be an issue with such a domain but only Cloudflare’s support can clarify that. Open a support ticket.