Often when users have this problem it's related to 'Settings' in either the Chrome Browser itself, or in the MacOS 'Privacy and Security Settings'. They will often receive a message that says "Message: Permission denied by system" when this occurs.
All you have to do is give Mac OS permission to let Chrome use your microphone / camera / screen recording.
On your Mac:
Click 'System Preferences'
Then click 'Security and Privacy'
Choose the 'Privacy' tab
Click the 'Lock' icon and enter password, if necessary.
Click Camera / Microphone / Screen Recording on the left
Add a check mark ✅ beside 'Google Chrome'
Save changes and restart your desktop/laptop if necessary.
If you're a participant, just go back and click the Participant Link your researcher sent you.
If you're a moderator, just go back to your Dashboard and click into the appropriate "Live session" or click the "In-person" button, depending on which test is being carried out.
You'll find that starting a test is a breeze!