This article explains how to disable the online mode and allow cracked players to join the server. You might need to do this if you see the error message "Failed to log in: Invalid session (Try restarting your game and the launcher)".
Players with cracked Minecraft cannot authenticate themselves to the Mojang authentication system, which is enabled by default on every Minecraft server. You can disable this authentication system by disabling the "online-mode", which you can find as "Cracked" in the options: https://aternos.org/options/
Disabling the Mojang authentication means that anybody can connect to your server with any name and even use your admin privileges. Therefore it is highly recommended to prevent this by installing an authentication plugin like SuperAuth.