This error may occur when you try to change your Primary Platform. When this occurs the Primary Platform cannot be changed.
To fix, unlink the desired Primary Platform account and launch Rocket League:
- Sign into your Epic Games Account
- Visit the Connected Accounts Page
- Select the account that you want to use as your Primary Platform Account
- Click Disconnect
- Launch Rocket League on the platform that you want as your Primary Platform
- Sign in to your Epic Games Account
- Select the account that you want as your Primary Platform Account
For more info check out: How Do I Unlink Platforms From My Epic Games Account?