What mac model and year is your mac, click on the top left of your screen and click on About This Mac.
macOS Big Sur is compatible with these computers:
MacBook introduced in 2015 or later
MacBook Air introduced in 2013 or later
MacBook Pro introduced in late 2013 or later
Mac mini introduced in 2014 or later
iMac introduced in 2014 or later
iMac Pro
Mac Pro introduced in 2013 or later
View the complete list of compatible computers.
If upgrading from macOS Sierra or later, macOS Big Sur requires 35.5GB of available storage to upgrade.
If upgrading from an earlier release like Mac OS X El Capitan, macOS Big Sur requires up to 44.5GB of available storage. To upgrade from OS X Mountain Lion, first upgrade to OS X El Capitan, then upgrade to macOS Big Sur.
As you can see from the above if you are upgrading to Big Sur directly from El Capitan you need at least 45.5GBS of free space.
Although Apple say you can upgrade directly from El Capitan to Big Sur I would not recommend it. Part of the installation is converting your file system from HFS+ to the newer APFS. There have been reports that upgrading from an early OS like El Capitan to Big Sur can result in the installation hanging and the mac becoming unusable.
I would recommend doing an intermediary upgrade to macOS High Sierra, this should provide the necessary firmware
to convert the file system to APFS.
For macOS High Sierra,
Open Safari and click on this link,
How to get old versions of macOS – Apple Support
Go to the Download OS section and click on macOS.
Before upgrading make a full back up of your Mac.
Check that third party apps you use have updates available for the new OS.
https://roaringapps.com
Check that your printers/ scanners etc have updated drivers and software available for the new OS.
Connect your mac to your router via cable rather than rely on WiFi.
Do not use your mac while it is downloading
and do not use any other devices that share the same internet connection.
If the App Store says ‘Open’ then it is detecting a previous download of the install app
go to the Applications folder locate Install macOS ******.app.
Move it to the Trash, empty the Trash.
Restart, click on the link again, the App Store should say ‘Get' or 'Download'.
If you want to or are able to install Big Sur, open Safari and click on the link below.
How to upgrade to macOS Big Sur - Apple Support
If you are considering upgrading to Big Sur be warned it does not support 32bit apps.
Your Mac is probably running 32bit apps and these will need to be updated to 64bit apps,
some updates may be free, some may be paid. Use this app to discover which apps you have
are 32bit, https://www.stclairsoft.com/Go64/ then contact the developers of the software to see
if they are offering 64bit updates.
Big Sur is a very different OS to El Capitan so make sure you are fully up to speed as to how this will
affect you and the way you use your mac.