I keep forgetting to do this myself, so perhaps this will help someone. If your Character BP is physically placed in the level, but your character keeps falling through the floor when you hit play (or in my case, falling through the landscape), go to the Details panel of your Character’s BP, and check to make sure “Auto-Possess Player” is set to Player 0. If it is not, then a lot of weird things can happen:
1. the Character will not spawn at the location it is placed in, but will instead spawn at a Player Start if one exists-- and if the Player Start is in a bad location, the character will fall through the floor.
2. The Character will spawn in some random location and you will have 2 duplicate characters in the level, one playable, one not, and if it’s a Metahuman you’re using, the playable character’s hair will be sticking straight out like a bush… LOL!
3. If you play in a Selected Viewport or a New Editor Window, the Character will appear to spawn a bit above the location you placed it but will fall a short distance and land on your landscape. However, if you play in a Standalone Game, the Character will fall through the landscape to infinity.