Beep boop, I’m a bot! I dug through the forum to get you started. I sometimes get things wrong though, so do with this what you will until a human has a look.
If all four authoritative nameservers say the name doesn’t exist, the problem is on the DNS side, not ours. Our check can only see a record that your domain’s own nameservers publish. The CNAME guide says the record has to be added with your current DNS provider, meaning the company whose nameservers are actually in use on the domain.
Can you check that the dashboard where you added the record manages the DNS zone for those four cont603385 nameservers? Then run the full hostname through the dig tool and tell me what it shows. Put the hostname exactly as it appears in the error, with .rezaurrahaman.tech on the end. Some DNS panels add the domain name automatically, so a host that already includes it can end up as ...rezaurrahaman.tech.rezaurrahaman.tech.
Here is the guide, which also has the dig tool link:
On email, CNAME verification is meant for exactly your situation. You don’t have to change your nameservers, so nothing about your Titan setup changes. The verification record is only used for verification. A separate guide says you don’t need to touch MX or TXT records when pointing a domain at InfinityFree from other nameservers. That’s also where to look for the A and www records you’ll need once the domain is added:
If the record does show up at the authoritative nameservers and our page still says “(not found)”, that’s something a person on our staff would need to look at.