The new "Lab of Legends" PvE lab is live, but some players are encountering a "Matchmaking has failed" bug when they try to play it. It seems this has something to do with the player loadout defaulting to invalid values; RiotExis provided a temporary workaround for this problem on Twitter.

Quote From RiotExis
Lab of Legends is now live! Come check out the latest evolution of single player adventure Labs in LoR.
 
If you get a matchmaking failure in the Lab, make sure you've selected a Board and a Guardian in your loadout! That should get you into game while we roll out a fix.

 

This is also a good chance to mention we've started a new guide for Lab of Legends -- we'll fill it out with information as we discovered it, so check back soon!