Checklist
- Invite only one bot
- Use a new, level 1 bot account
- Queue into BR or Zero Build duos, trios or squads
- Set matchmaking to No Fill
- Avoid Ranked, LTMs, Fortnite OG and creative modes
- Wait for the bot to fully load in before readying up
Why it happens
A bot lobby needs a single brand new bot as the matchmaking anchor. If the bot is not new, there are extra bots, matchmaking is set to Fill, or the mode is unsupported, matchmaking falls back to a normal game.
The bot also enforces some of this for you: it turns fill off when it owns the party, warns about extra bots, and refuses unsupported modes.
Try again
Back out of the match and rebuild the party, waiting until the bot has fully loaded before readying up.
More detail: How to get into a Fortnite bot lobby.