Most of the time, this "You must be a pickup member" error gets generated under the following scenario.
User signs into the application. They see a call in queue in the lower left corner but it is not ringing them as an operator.
(but it is not appearing in the Audio Conversations window #1)
So, they click on the green answer button next to the call in queue. (#2)
Really, the issue is completely different.
If the operator does not have a valid microphone, or access to it has been denied in the browser, RingCentral does not consider you a valid endpoint.
This means RingCentral will not send a call from the queue to the console.
AND, if they do not have a valid mic, they cannot Pickup a Call from queue, even if they are a member.
The first test is to verify that the user is able to place a call and speak to another user.
The easiest way is to just find a user and click on the Call button. If this does not work, neither will any sort of answering technique.
If you are truly able to answer calls, but are trying to "cherry pick" or pick queue calls out of order, or while signed out of the queue, then the following applies.