Sometimes, your antivirus software is the culprit. Certain antivirus programs falsely identify the steamapi64.dll file as a threat (a "false positive") because it interacts deeply with the system kernel for DRM (Digital Rights Management) purposes.
Here’s a of the most common fix for the steam_api64.dll missing error in Black Ops 3 (verified solution), followed by a quick verdict. Sometimes, your antivirus software is the culprit