It can be very difficult to get screen connect going on a Mac using Safari.
If the customer has Chrome or Firefox skip to using it. It may also save time to quickly install one of these browsers for them and then do the screen connect.
For example a ticket was escalated where they tried everything to get Safari to screen connect and failed. I skipped to installing Chrome and then tried screen connect. We had to hit “try next option” until we were prompted to “Launch Application” and then screen connect worked fine. The “try next option” link usually doesn’t even show up in Safari so it can be a waste of time to try to get it going in Safari.
On modern OS X systems the ScreenConnect client will be blocked as an unknown developer. Simply go to the Apple\System Preferences\Security&Privacy and on the General tab that comes up you will see an “open anyway” button that will allow the app to open.