Fortnite surprisingly does not feature a practice mode where you can spawn bots to play against. The Battle Lab doesn't give you the ability to make a real game with bots, but there's a trick lets you play in a bot only game.
How to Play in Bot Only Lobbies in Fortnite
When you make a new account for Fortnite, the first games you'll get into are against bots. This is so new players can get a feel for a game before the serious competition starts. The bot lobbies will also work when a new player is on a Squad with other players, which is what this trick exploits.
To play in a bot only game, you need to make another account. You'll also need another device to play on. Simply join the same party as your starter account and queue into a Squads match, making sure you toggle No Fill.
As soon as you and your second account get into the game, make the starter account leave the lobby. It's important that the starter account leaves the lobby before it earns any XP so it remains a beginner as the game sees it. From here, your main account gets to roam the island against bots.
The bots are painfully easy to fight against compared to real players, but there is some value in playing bot only games. Bot games let you explore new locations and try out new weapons after an update.
You can complete challenges without fear of being disturbed, as those are still tracked. The usual NPCs will still spawn, letting you complete challenges that you wouldn't be able to in Team Rumble. This is most helpful for challenges where you must eliminate a Boss enemy, as the toughest part of those challenges is fighting off other players vying for their weapons.
Since bots are more predictable to fight against, you can use bot games to help you practice your fundamentals like building and shooting if you're still in the early stages of your Fortnite career.