This is the most reliable fix. It forces Steam to scan your installation and re-download any missing or corrupted files, including the missing DLL.
typically stems from antivirus software flagging the file as a "false positive" and quarantining it. While the standard file for Steam is steam_api64.dll , the specific steam-api64r.dll (often with an 'r' suffix) is frequently associated with specific game versions or repacks. Common Solutions call of duty infinite warfare steam-api64r.dll missing