The directory could not be completely unmounted. This is usually due to applications that still have files opened within the mount directory. Close these files and unmount again to complete the unmount process.
This error happens when you use another Dism.exe which is located in C:\Windows\System32. The correct application to use is C:\Program Files\Windows AIK\Tools\amd64\Servicing\Dism.exe, so you need to direct your command line tool to that location before you run the unmount-command (use cd C:\Path to change directory). Replace amd64 in the path with your architecture or leave it out to go in the Servicing folder inside Tools.