Why won't Big Sur download?

..Perhaps the initial start of this process was not properly engaged.

Was the 'prompt to upgrade' from older system version a clear path?

Did you try Safe mode to help use the installer once downloaded?

• How to use Safe mode on your Mac - Apple Support

https://support.apple.com/en-us/HT201262

Is your Mac listed among those supported for this upgrade?

• How to upgrade to macOS Big Sur - Apple Support

https://support.apple.com/en-us/HT201475

• macOS Big Sur is compatible with these computers

Learn how to download & install macOS Big Sur. Confirm compatibility before downloading.

"If a macOS installer can't be used on your Mac, the installer will let you know."

The article above plays details fairly well:

"If you're using macOS Mojave or later, get macOS Big Sur via Software Update:

Choose Apple menu  > System Preferences, then click Software Update.

Or use this link to open the macOS Big Sur page on the App Store:

Get macOS Big Sur. Then click the Get button or iCloud download icon."

Older than Big Sur are listed here:

• How to get old versions of macOS - Apple Support

https://support.apple.com/en-us/HT211683

• How to create a bootable installer for macOS - Apple Support

https://support.apple.com/en-us/HT201372