Thrillseekers is the newest Apex Legends event to come into rotation, and it is themed entirely around Arenas. That said, you would imagine even though Arenas is the focus of the event, that players would still be able to play Battle Royale mode as well. However, due to a glitch in the even release, no players could queue into Trios or Duos.
The reason behind this is that the game glitched in away that it is still holding the old maps from the Legacy Maps Takeover for the Genesis Collection Event, while trying to look for the current maps, and it left players with a blank screen tab, with two locked black boxes. This is happening across all platforms. That said, if you are eager to play Battle Royale, since it was untouched by the event, Ranked Leagues still works fine. Arenas works as well, but is currently still the Legacy Maps rotation, including Skull Town.
The developers know about the problem and are looking for a fix right now. The fix will again, not touch anything in Ranked, but fix the Arenas playlist rotation options, and add Trios and Duos back into the game.
The Thrillseekers event Launched July 13 and goes until August 3.