WHY DOESN'T mutex WORK HERE? [C++]

Question

I tried to set up mutex and it doesn't work. Any clue?
https://code.sololearn.com/c2xMUbMr8yIR/?ref=app

Answer 1

https://code.sololearn.com/clN62FLG2CY3/?ref=app
explanation why it doesn't work is in the code comments:
"NOTE: THIS WILL NOT RUN HERE ON SOLOLEARN. Sololearn's compiler doesn't support threading on Windows (which is the OS their server uses). You'll need to copy this code into your own IDE to try it out."