Fatal error when trying to uninstall Test complete

Open a Task Manager and see if you have an older version of TestComplete still running. Additionally you may want to check you registry keys for any reference to the older version of TC. Although I can't recall if the error I received was the same as yours, I did experience a similar upgrade issue when moving from TC 14 to 15 and this was the workaround.

Computer\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Uninstall

right-click on the 'Uninstall' folder and then Find 'testcomplete' and delete any folders referencing the old version. If you aren't comfortable using regedit.exe, or don't see any old versions, just wait to see what SmartBear says.