Cloudflare Origin Certificates Error on SSL/TLS

I have a domain here
I have added my domain to Cloudflare and want to add a Origin CA Certificate on website, but whenever I try to add the Certificate Information it tells that certificate is not valid for domain just to try if all works well, staging website).
Why is this happening, why can’t I add a SSL/TLS Certifacte of my choice from Cloudflare.

TechDroid ROOT

You can upload a certificate from any SSL vendor here, but you can only use SSL certificates which are generally accepted by browsers. Cloudflare’s Origin Certificates are signed by Cloudflare, and Cloudflare itself is not a generally accepted certificate authority. So essentially, Cloudflare’s certificate are self signed just like the certificates we enable on all domains by default.

That means you cannot upload Cloudflare’s origin certificates here. You can set Cloudflare to “Full” mode (not strict) and use our default certificate, which also ensures that the connection to your website is encrypted all the way. Or you can purchase a certificate from a recognized SSL certificate vendor to use “Full (strict)” mode.

Can you tell any other SSL certificate providers which are free that can be used with Infinity Free.

TrustAsia I think offers free ssl but I am not sure if they are trusted by major browsers