Hello ,
Thank you for posting in the community. The cancel button should work but could be slow to respond in certain instances but should work to cancel after waiting few minutes. I recommend you force quit Twinmotion from the Task Manager if necessary.
If you could also submit a bug so we can properly investigate this behavior that would be very helpful. https://twinmotionhelp.epicgames.com/s/case-community-page
Kind regards,
Vincent B.