Currently, there is no direct way to Rocket League Trading download Rocket League from Steam, meaning you can't just search it on the store and install it in your library. This is because Psyonix, the developer of the game, was sold to Epic Games a few years ago, which caused the game to be removed from Steam.
If you had previously purchased the game on Steam before the Psyonix sale, you can still play the game on the platform and install it from your library with ease. The Steam version is still fully supported too, so you don't need to worry about the game not being up-to-date on the platform.
The only way you can currently get Rocket League on Steam is by purchasing a key from a third-party key reseller. Many of these sites can be ethically questionable (to put it lightly) so we won't be linking to any here, but you can easily find them with any search engine.
Another workaround you can do is install Rocket League from Epic Games, and then add it to Steam as a non-Steam game. This can be done by opening Steam, selecting "Add a Game" from the lower left corner, clicking "Add a Non-Steam Game", and then locating Rocket League in the file browser. You won't be able to Rocket League Trading Prices track playtime or achievements with this method sadly, but it helps you keep your games centralised to Steam if that's what you want.
