You need to add a TXT record for _railway-verify.rentmate.bd. If you are using the API to add your custom domain, you can find it in verificationToken under status.

Have you tried the suggested solutions by the agent?

yes, but didn't solve

I did some research and testing on this issue. I tried to add your domain, but failed as well. And upon searching I found that direct registration at the .bd second level (e.g., example.bd) is not permitted for most general users. All domain registrations must be at the third level under specific second level labels.

So this is a limitation by the bd TLD nothing related to Railway. Your options are either use a subdomain (e.g., www.rentmate.bd) or use a different TLD (something other than .bd).