steamclient64.dll is a core component of the Steam client, responsible for handling user authentication, game ownership validation, cloud saves, Steam achievements, networking, and DRM (Digital Rights Management) verification. When an application built with Steamworks SDK attempts to initialize the Steam API, it calls SteamAPI_Init() , which triggers the loading of steamclient64.dll (or steamclient.dll for 32-bit).
Antivirus software often falsely flags steamclient64.dll as a threat and quarantines it. Check your protection history and restore the file, then add the game folder to your exclusion list. unable to load library steamclient64.dll