(Refer to the 2 images at this site: https://drive.google.com/drive/folders/16BoMd9-6ntILMYJFVSjRnfeB4ITV-3nQ?usp=sharing)
I decided to upgrade from 21.04 to 21.10. I used the update-manager -c method of upgrading. After 12 hours, I checked and it was yet running and, from the messages on my terminal screen, appeared to be without issue.
After another 5 or 6 hours, I thought it was done. That's what is reported on the terminal window in image 2.
However a new window popped up entitled, "Remove obsolete packages?" Also on that window are details about obsolete and to-be-removed packages... but, (as you can see in the first image), there is no UI element to allow me to respond.
The process was on the step just before the "reboot" step. I fear that aborting will render my machine demolished...
Why does the 21.04 -> 21.10 upgrade fail to allow me to respond in order to proceed?
Stopping the system while packages are installing may indeed destroy your system.
However, stopping the system while packages are downloading, while snaps are updating, while obsolete packages are being removed, or while the system is idle is likely to cause only minor damage that --in most cases-- can be readily repaired.
If possible, do try to avoid interfering with the upgrade. But if the upgrade seems be stuck then determine which step you are stuck upon.
Even though there was no indication of any GUI response elements, I tested my hypothesis that it was just a poorly-designed screen... by making sure that the last item in the GUI box (displaying the various packages to be removed -- as it said -- either obsolete or no longer needed) was selected.
Then I tabbed watching to see if the topmost element on the page was selected and it was not. I tabbed again... still no response on the screen. The third tab took me to the first item at the top of the screen.
That confirmed my hypothesis that "invisible" elements (like "OK" or "Cancel" buttons) were, indeed, on this screen. Attempting to change the window size (in the hope of revealing the missing elements at the bottom of the screen) failed. It (the window) had been designed and/or written to be of fixed size.
I was very happy to have learned of the cause... so, I guessed, the second of the 2 missing elements should be the "OK" button (although, these days I find that it appears first of the 2 (OK, Cancel) about 10% of the time.
So, when you are asked a question in a dialog box and you see no way to answer said question, try exploring to see if the missing items are, in fact, present.