I had the recent experience of Camtasia throwing the same error when I tried to open a TREC file I'd just recorded. I got nowhere with Tech Support, but rebooting my computer allowed me to read the file.
See if you can play the file in a media player. If so, I suspect it's an issue of Camtasia Recorder keeping the file locked, and Camtasia not wanting to open a locked file for editing, and a reboot should fix it. If so, please report this to tech support so they will finally fix this problem, which has been an intermittent issue with Camtasia for years.