If you encounter the Rocket League version mismatch error after updating, the first thing you should do is turn off your console or PC. Once you turn it back on, try opening the game. Generally, shutting down your system should resolve the issue, but if you still see the error after this step, restart your platform (via At the start of the game).
If all else fails, you should check if your copy of “Rocket League” is up to date. If you’re using Xbox and PlayStation consoles, just go to your games folder and check if the game needs an update. On Steam, go to the game’s official page and see if it offers an update request. The same process should apply on the Epic Games Store. When it comes to Nintendo Switch, it could very well be that you don’t have enough space on your device to complete the latest Rocket League update. Delete everything extra that exists in the internal memory, reinstall the update, and you’re done.