How to Replace nameservers

[KangJL]

So you are using free subdomain?
If that is the case, you will not be able to use CloudFlare.
Custom domain is needed.
See below for more info

Where do I get free domains?

You can try using EU.org or PP.UA (the latter one if you have a credit card even with a few cents on it and a phone number to use for verification, though the credit card is used only once for verification and then what’s free will remain free) as a valid alternative to Freenom (and Dot TK because Dot TK and Freenom are the same) that also supports Cloudflare and other things that Freenom with its domains supported.