Domain does not work

DNS, HTTPS, Cloudflare errors, browser warnings

The domain does not open

Right after purchase – the domain does not point anywhere yet. Add an A record, a CNAME, or NS: Connect.
After a DNS change – make sure you change records where the domain NS point, that the main domain has an A, AAAA, or CNAME record, and that the name is @, not the full domain.
www does not open – it needs its own record.
Changes are not visible – check the record in DNSChecker. If some locations already see the new value, wait for caches to update. After 24 hours with the old value everywhere – contact support.

521 – the web server is not running, not listening on the port, or blocks Cloudflare IPs.
522 / 524 – the server is not reachable from outside or responds too slowly.
525 – HTTPS does not work on port 443 of the server.
526 – with Full (strict) the server has no valid certificate for the domain.
Too many redirects – with Flexible the server itself redirects to HTTPS. Remove that redirect, or install a certificate and switch to Full (strict).

Google – it is a label on the website content, not a DNS issue. Open Security issues in Google Search Console, remove the malicious or deceptive content, fix the vulnerability, and request a review. Google guide.
Cloudflare, or "Blocks: present" on the card – contact support.

Changing DNS does not hide a label: remove its cause first. How to lower the risk in advance: Ban protection.

@lumisup_robot: the domain, the exact error text or code, what changed before it, the current NS, and a DNSChecker screenshot if it is a DNS issue.