Unable to add Free domains to Cloudflare (.tk , .ml , .ga , .cf , .gq )

That domain is 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.

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