There's 2 reasons why you can't record: either Chrome settings are not properly set up, either Mac OS is preventing you from using the mic.

You'll find below fixes for both cases.

Step I - Allow the microphone in Chrome

That should solve everything.
If you still have problems, your operating system must be blocking the mic.
Follow the second step to fix this problem.

Step II - Allow the microphone in Mac OS

To check the box:
- click on the cadenas at the bottom left
- enter your password
- check the box next to Google Chrome

Now you should be good. Enjoy Teachee :)!