Here's the solution that worked for me so hopefully someone may find this helpful. The Windows 11 installation was failing because one of the driver folders in C:\Windows\System32\DriverStore\FileRepository was corrupt and/or incomplete.  Almost by definition (as long as your pc is running fine beforehand) it's not something you need so can be deleted. To find which folder to delete you need to download and run setupdiag.exe from here [URL] Once run it produces a zip file which contains a text file called setupact.log. Open it and go to almost the end and find the word "error" which will be at the start of a line which shows the offending driver folder. To delete the offending folder you will probably first need to take ownership of it from the system as otherwise it won't let you do it. Google is your friend here. For me, having done all that, I restarted and the Windows 11 update worked.
I've joined the forum in the hope that someone can help. I have done all the checks that my laptop is Windows 11 compliant - both the Microsoft tool and finding it on the list that Dell have said is OK. Every time I download Windows 11 from Windows update it goes through the install process fine and reaches the point where I need to restart.  On restart it reaches 1% installed and then seems to try another restart.  At that point it just brings up a message saying it's undoing changes made and reverts back to Windows 10.  This has happened about 10 times and is driving me mad.  Does anyone from Dell read this?  Is there some sort of tool you have to work out why this is happening? Fingers crossed someone has a solution. Thank you.
Inspiron 7591 2n1 will not upgrade to Windows 11