Big Sur download failure and iMovie

Try the following procedure to obtain the highest iMovie version that Mojave will run. It would require that you had iMovie 10 on your Mac at one time, even if you perhaps deleted it. Your 2017 MacBook Pro should have come with iMovie installed, unless you bought it second hand.

To update/redownload iMovie 10, drag your iMovie 10 app from the Applications folder into the trash, but do not empty the

trash. (You can skip that step if you had iMovie 10 on your computer before, but deleted it.) Sign in to the app store and open your purchases folder by clicking on your name at the bottom of the app store side bar. You will see your purchases displayed, including iMovie 10. Redownload/update iMovie from there. If not successful, drag your present iMovie app out of the trash (if you moved it there) and return it to the Applications folder.

I don't know why Big Sur won't install on your Mac. You might want to contact Apple Support and talk to the techs there. They have the capability of remotely accessing your computer that we don't have here. To contact apple support click on the

Contact Support item in the upper right hand corner of this forum’s screen.  Navigate to a place where you can enter your phone number.

-- Rich

Best of luck in getting this solved.

-- Rich