Read the “cannot practice right now” card
The screen usually shows one reason card. Used-up planned time, an expired trial, or a class that is not open is not a microphone problem. Follow that main reason; do not stack several guesses.
The microphone itself
Allow the site to use the microphone, confirm the system input is not muted, and quit meeting apps that hold the device. Switching computer or a newer browser often helps more than refreshing.
Still blocked
Tell the teacher the approximate time, browser, and task name so the teacher or organization admin can continue. Do not change the organization code or register again. Troubleshooting is in Help Center.