That domain is probably registered under a TLD which is governed by the Freenom registry. Freenom currently experiences issues with their Whois service which prevents such domains from being added to Cloudflare. Freenom needs to fix that on their side. Unfortunately there is no ETA known for that.
It probably is best to register a proper domain instead. They already go for less than $10 a year (even as low as a dollar for something like
.xyz for the first year). If you’re in education you could even get a free one for a year with Namecheap or Name.com