The requested top-level domain is not supported; this tool supports all delegated gTLDs contracted with ICANN and their corresponding second level domains.

How is this related to Cloudflare?

I’m trying to activate the domain with the .it extension

IT domain does not have DNS terms

I am not sure what the issue is. You add the domain to the service as described in the documentation, change the nameservers, and are good to go.