Big Sir will not download

There is NO problem with my internet. 

There may be many problems even when some connection finishes successfully on the internet.
How have you checked there is NO problem?

It's better to download Big Sur, not Big Sir… 🙂

When does it fail ? At beginning ? At other stage of download ?
Do you get an error message ?

How much disk space available ?

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.

What is your present MacOS version ?

If it is an old one, you could try to update first to 10.15.7, then to Big Sur ?
See:
https://support.apple.com/en-us/HT211683

My current iOS is 11.0.1