It says "Connection not ready" or will not connect
It is almost always the file name. If your browser renamed it to rustdesk.exe or download(1).exe, it lost its configuration. Download it again from your Eli-T system, under Help → Install Remote Support, and check that it keeps the full long name.
Black screen on a Mac
Screen Recording and Accessibility permissions are missing. Turn them on in System Settings, under Privacy and Security, then open the program again.
I already had it installed
Old configuration may be left over. Close the program, delete the %appdata%\RustDesk folder on Windows and open it again.
The technician connects but cannot do anything
On a Mac this is usually the Accessibility permission. On Windows, if the technician needs to touch something in the system, the program has to be opened as administrator. Tell us what you see on screen and we will walk you through it.