That is likely the cause of this error. You need to go to your Aternos player settings and manually allow you or your friend into the game by adding their in-game username in the “Whitelist” section. Alternatively, you can also close the whitelist option altogether. Though, that might ma...
Now that you've created your Realm, chooseConfigure Realm(the wrench icon) and click onPlayers. Now all you have to do to whitelist a friend on your server is clickInvite playerand type in their username before clickingInvite playeragain. Your friend will receive an invitation to join your ...