I installed TeamViewer 12 on my Linux system, it seems to run regularly, but I didn't try any real remote control. I saw a video tutorial in which the operator shows it is possible to simulate a remote control by inserting the target ID "12345". In that video a new window with the remote system appears and as I well understand it should be a real remote machine provided by teamviewer to let users test their installed software.
I tried to insert "12345" as remote ID, but it returns that ID is unknown.
So I asked if that testing remote ID is still available or not, and if it was really available in the past.
The test ID 12345 is not available anymore. That's the reason why you're getting that message. If you need to test TeamViewer on your system, we recommend you to install TeamViewer on another device and connect to that one.
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