Is this a 2020 Intel Air or a 2020 Apple Silicon Air?
Is this is a 2020 Intel Air and you upgrade to Tahoe, you likely will wish you did not. Performance will be terrible, especially if this is an i3 model. If this is an Apple Silicon, and your workflow in general supports it, I would recommend updating to Sequoia or Tahoe as these will be supported operating systems for a few more years and both will support Microsoft's latest releases. Microsoft follows Apple's lead of current - 2 support. For example, Apple supports Tahoe, Sequoia, and Sonoma. Later in the month when Apple releases Golden Gate, Sonoma will be dropped from support. Usually by November, Microsoft follows by dropping support.
Here are a few things to try.
The basics:
• Are you logging into the Air as an admin account?
• Do you have enough free space to download the updates?
• Do you have software that is forcing you to stay on macOS 11?
• Is the device managed by an MDM?
If the answers for above are yes, yes, no, and no, then try these steps:
1: Kick software update in the head by:
• Open Terminal
• Run the following command
softwareupdate --list
• Do you get a result? Relaunch System Preferences and try Software Update again
2: Manually download the OS update you want to upgrade to and attempt to install from the installer.
• You can use the App Store versions by using the links in this KBase article: How to download and install macOS - Apple Support
• Or if you know which OS you want, you can grab the Install Assistant package direct from Apple using one of these links:
For Tahoe 26.6.2: https://swcdn.apple.com/content/downloads/37/33/140-93587-A_GRFFH93NOL/f944yaqo1cjhh2m0kxrl0zhcpg9yb9qphv/InstallAssistant.pkg
For Sequoia 15.7.9: https://swcdn.apple.com/content/downloads/16/08/140-85388-A_3LX3Q36I6P/zxod6nr7zkovvsml6s628z44gdbc4exa5p/InstallAssistant.pkg
In either case, this will download a package installer titled InstallAssistant.pkg. Double click it to install the installer. The PKG will unpack an installer into the Applications folder.
Go to the Applications folder and run the installer.
3: Boot the machine into Safe mode and then try to upgrade.
Apple has details on how to do this here: Start up your Mac in safe mode - Apple Support (IN)
Let's start there. Hope this is helpful.