KB ID 0001633
While attempting to uninstall Microsoft Exchange server;
Setup can’t continue with the uninstall because the powershell (PID) has open files. Close the process, and then restart setup.
Seems to be a common error, and is usually caused because someone has the Exchange Web Management page open, (probably in another user session);
In some cases you may need to reboot, but in my case I was simply being a doofus, look at the window I’m running the command from! Open an administrative command window and try again 😉
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