The domain nabilreza.com is not on any Railway project, so the "taken" message means it is already registered at another registrar globally. Railway's domain search checks real-time registry availability and can only register domains that are currently unregistered. If other sites show it as available, they may be offering it as an aftermarket/premium purchase from the current registrant rather than a fresh registration. You would need to acquire it through whichever registrar or marketplace currently holds it, then add it to your Railway service as a custom domain.

It is not registered with another registrar globally. I still find it with another hosting platforms.

A taken result in our domain search means the domain is not available to register through us, not that it has been registered somewhere else. When another registrar can sell a name that we cannot, the path is to register it there and then connect it to your service as a custom domain.