I am unable to download Big Sur to myMacBook Pro (Retina, 15-inch, Mid 2015). Help? The link on Apples website does not work

use these links instead of trying to obtain macOS directly from the Mac App Store:

How to download macOS - Apple Support

Rremember that your new download will be tagged wit the Apple-ID used to obtain it, and if you have changed to a different Apple-ID than previously used, that breaks the chain and may make future upgrades much harder to obtain.

What is happening when you try to download it? It can be that it won't let you download It because of your MacBook is capable of running macOS Monterey. Try and download Monterey instead if you're keep facing trouble when downloading Big Sur