The same issue failed to download. My internet is fine. iMac 27, 2017

Big Sur starts to download and gets about 10 seconds in, then an error message appears telling me to check my internet. I checked my internet by downloading Big Sur onto a different computer on the same connection and it worked fine. I have over 500 gig disk space available. I spent some time with a chat window with apple support on Friday. After trying everything I have tried in the past, she told me to uninstall and reinstall my operating system. But I need to back up the drive first. So that is my next step unless someone else has a simpler idea.

My current iOS is 11.0.1