Your certificate could not be chained to a valid, trusted CA root, li

[KangJL]

@keegang

it mean i can’t even find cert and install by my self?

Correct.
Only way to make it work is to get your own custom domain and use CloudFlare on free hosting

To get your own domain, read below

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.