Apple are making their error messages as obscure as windows ones.
I would do the following.
1. Reset the TC to factory settings..
2. Make sure no computer is using the TC disk.. Apple state this condition for upgrade.
That would include even mapping the drive.
3. If you finally cannot get it to go, locate a MacBook equipped friend to come and have a drink.. bring laptop.
In the end the TC is designed for Apple, by Apple.. with the merest nod to windows. When in doubt you need a Mac.