What is the name of the domain?
What is the error number?
HTTP Status: 404 (not found)
What is the error message?
HTTP Status: 404 (not found)
What is the issue you’re encountering
Domain not working after adding nameservers
That 404 page is from your origin/web server behind Cloudflare. You can tell off the response headers/the existence of CF-Cache-Status, the non-cloudflare normal x-request-id response header or that it’s not Cloudflare branded, that it came from your origin.
You’d have to investigate more with your origin to figure out why. One possible thing that it’s worth ensuring you have set right anyway is your SSL/TLS Mode, under SSL/TLS → Overview (Magic Link: https://dash.cloudflare.com/?to=/:account/:zone/ssl-tls. Switch it to Full (Strict) if it’s Flexible or Full. Otherwise, check with your origin/web host.