"S1" is the internal codename for the game, "mp" stands for multiplayer, and "64" indicates it is the 64-bit version of the shipping (final) build.
Then that game folder has been tampered with. The file is not part of the original game. Delete it, then verify game file integrity via Steam/Origin/Epic Launcher. Also run a full system scan, because the injector likely modified game DLLs. S1-mp64-ship.exe -
: If the file is missing or permanently damaged, a clean re-installation of the multiplayer component is usually required. Security Check: Is it Safe? "S1" is the internal codename for the game,