You need to have the mission or Nano mission to enter. Portals can only be entered that way. If it's a regular Lair, you need enough Fusion Matter to unlock the mission. If it's a Secret Lair, you needs to find the mission that takes you in and be the correct Level or higher. If you already have the mission, then that's a different problem. Did you do what the mission is telling you to do. If you did, then it's probably a glitch or bug.