Mid-2015 Macbook Pro can't be updated to MacOS 11

I have tried (and failed) numerous times to update my mid-2015 MBP (11,4 w/16GB RAM, >300GB free SSD space) from 10.15.7 to 11. The same thing happens every time: the installer begins the install process, it reboots (as it should), but then, when the progress bar gets to about 10%, the Mac reboots. The process repeats infinitely. I've tried about six different install apps from the App Store and other sources. I'm stumped.