A corrupt file in the Rocket League Cache is the most common culprit of these error messages. Deleting the cache typically fixes this error.

1. Click on the Folder Icon in the lower left corner of the screen
   This will open File Explorer
2. Click on Documents in the left-hand column
3. Double click on My Games
4. Double click on Rocket League
5. Double click on TAGame
6. Right-click on Cache
7. Select Delete

Try starting Rocket League again

If Rocket League still won't start, try renaming the Rocket League folder. In order for this to work, Cloud Sync for Rocket League will need to be disabled. This prevents Steam from copying a corrupted version of the folder back into the game files.

First, disable Cloud Sync in Steam:

1. Right Click on Rocket League in the Steam Library
2. Select Properties
3. Click on the Updates tab
4. Uncheck Enable Steam Cloud Synchronization For Rocket League
5. Click the Close button

Next, Rename the Rocket league folder:

1. Click on the Folder Icon in the lower-left corner of the screen
   This will open File Explorer
2. Click on Documents in the left-hand column
3. Double click on My Games
4. Right-click on Rocket League
5. Select Rename
6. Type Rocket League Old
7. Tap the Enter Key on your keyboard

Try starting Rocket League again