Here is informative text regarding the file steam_api64.dll in the context of WWE 2K15 .
(rarely needed)
The "64" in the filename indicates that this version is for 64-bit systems. Since WWE 2K15 is a demanding title, it requires the 64-bit version of the API to access more system memory.
If the above methods fail, the nuclear option is to uninstall and reinstall WWE 2K15. This ensures a completely clean slate. While it takes longer due to download times, it guarantees that all game files, including the DLL, are correctly placed in the directory.