How to find out program preventing windows shutdown

[ ]

You might try a scan with Adwcleaner
AdwCleaner Download
and see what that comes up with.

You could also compare shutting down from safe mode and see if you get the same message. If not, you could proceed with diagnosis from a clean boot, assuming that if you did a clean boot, you did not see this message when you shut down. That would work if this is a well-behaved program.

Note: if this is adware or malware, you will likely need a specialist tool, which is why I suggest Adwcleaner first. If that finds nothing there are others you can try.