'request permission to unmute' doesn't work.

Hello Brume,
You may "Ask to unmute" and will require the use to accept the prompt so they can unmute themselves. 
Additionally you may also enable "Request permission to unmute participants" in your account settings. You would then then need to select this option when scheduling the meeting. Meeting participants will receive a prompt as they join and ask for permission to allow the host to mute or unmute them.  Permissions, once given, will apply in all meetings scheduled by the same host. So eventually, you will still need the users to allow this permission in order for this function to work. 
Following are 2 support articles for your reference. 
https://support.zoom.us/hc/en-us/articles/203435537-Muting-and-unmuting-participants-in-a-meeting
https://support.zoom.us/hc/en-us/articles/360061059792-Requesting-permission-to-unmute-participants

Good luck!

I also ran into the same problem, although I followed all the steps in the support articles. 

I found that this feature only applies to meeting attendees who have signed up for a Zoom account (free), meaning it doesn't work for those who have just clicked on a meeting link.

I ran into this problem with attendees who join the meeting from their Amazon Echo Show.