Sometimes without a good reason F-Secure anti-virus software might block Steam games from launching and without a valid reason to do so.
When that happens you will get this error message when trying to launch a game that is being blocked from launching by F-Secure.
Steam – Game Launch Failed Rocket League failed to start (error code 51).
Another error code that you might get would be something like this.
Application load error 3:0000065432
If you are using F-Secure and see that error you must either turn off Deep Guard or add the Steam/game folder to excluded list.
F-Secure also sometimes falsely marks some .dll and .exe files from Steam games as viruses so you might want to add Steam folder to virus scan exclusion list as well.
Updated on 10.05.2021 with new error code.