I added my subdomain to the main domain of the website under the Cloudflare DNS app, using the A records but unfortunately the site has been showing Error 404 for over 3 days now.
I have just checked the DNS Checker and it has propagated. Please what do you suggest I do?
Would you share the subdomain here?
That is not a DNS response. It comes from the webserver running on the IP that your hostname (or subdomain) resolves to. If your hostname is 
Proxied, the 404 response is being retrieved from your origin server and presented by the proxy, but it still orignates on your webserver.
It may be useful to set that subdomain to 
DNS Only while you determine the problem on your origin webserver.
https://blog.immangelcfoundation.com/
Thank you for this. Will try it out
The site is loading now but saying correction not private.
Is it the server or Cloudflare that’s the issue?
That is your server. You need to secure it with a certificate. There are many options, including a commercial certificate, an automated free certificate, such as those offered by Let’s Encrypt, or, one of the most reliable options is to use a Cloudflare Origin CA certificate. Once you have it installed, itb will secure the traffic between your server and Cloudflare. It is only recognized by Cloudflare, so you will need to switch back to Proxied once you have it installed.