Good afternoon, @karina ...
I just tried running the Missing Assignments report in New Analytics in my own sandbox course. The report successfully ran, and it downloaded to my "Downloads" folder on my computer as a *.csv file. Have you tried using a different browser (such as an updated version of Mozilla Firefox or Google Chrome) to see if that might help solve the issue? Another thing to try would be to clear your browser history/cache files, close the browser, and then re-start the browser to see if that helped.
If none of those options work for you, I would recommend that you reach out to the folks at the Canvas Help Desk to see if they might have any other options for you to try.
Please keep us posted on your progress! Thanks much!