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).
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.