The 1st connection to a mac when a tech is trying to connect is somewhat confusing. THe first dialog the client gets when a tech send the "Screen Sharing" command is
THe answer to this is DENY, we do not need the "Remote Support Customer Client" to be able to connect with them. If you open system setting and try to allow it, you will be prompted for an Administrator. We dont need that, have them hit cancel if they have opened system settings. Have client go to the Grant access in the background of this screen shot. This will grant "Screen Recording" which is really just screen sharing rights.
After this it should ask to reconnect to the session. Once reconnect happens, you may see that you cannot use the mouse, in the upper left corner of the Rep Console session select, toggle control of customers mouse/keyboard. If this does not help, close the session and send them a new session.