Good day! 
You must be joining one of the “beam \ \ cruise” servers. That is their policy, the decision to refuse connections to new players. See their Discord for per-server information. You may also find other servers on BeamMP that do this, sometimes for obvious reasons, but in my humble opinion - unnecessary. There are other means to moderate or manage who gets to do what. Well, restrictions are always restricting, unfortunately! 
Drive safe and be well! 
Do you happen to know how long this lasts?
AFAIK their Nordschleife Sim Racing server doesn’t have a whitelist. Otherwise it’s for an idefinite amount of time. I think you do need to join their Discord in order to join their server though. Unless I’m wrong and they determine the time since your account registration date or something. I don’t know, really, I’ve long since migrated to beam# servers. 