Browse to your Just Cause 4 installation folder (usually found under C:\Program Files (x86)\Steam\steamapps\common\Just Cause 4 ). Add the folder to the exclusions list and restart your PC. 5. Opt-Out of Steam Beta Programs
This is the most direct fix for problems with the API file itself. It's a quick, safe, and highly effective procedure. just cause 4 unable to initialize steam api
Changing your beta status can force a client refresh that resolves initialization bugs. Browse to your Just Cause 4 installation folder