Try reinstalling the game to fix this problem. The program can`t start because steam_api64.dll is missing from your computer. How to fix steam_api.dll, steam_api64.dll, GTAVLauncher.exe for Grand Theft Auto V The application was unable to start correctly GTAVLauncher.exe. The application was unable to start correctly steam_api64.dll. The procedure entry point SteamAPI_RestartApp could not be located in the dynamic link library steam_api.dll
Another way to fix these errors is to download and install this and this.
If you get the same error even with antivirus stopped, you must check your hard drive for errors with this because something is not right with your hdd or with game archive / dvd. steam_api.dll is missing error, steam_api64.dll is missing error or GTAVLauncher.exe is missing ? Then maybe you need to reinstall the video game and if is not working, try to reinstall game with antivirus turned off and internet connection closed.