What happens when you try to update to Big Sur exactly? Does it issue an error? Or can you simply not find the download for Big Sur?
If it issues an error, tell us what it says.
Otherwise see the following support document for a download link for Big Sur.
click here ➜ How to download and install macOS - Apple Support
The above post by HWTech is incorrect. 10.9.5 Mavericks has access to the App Store, and the upgrade to Big Sur is supported on it according to Apple.
click here ➜ macOS - How to Upgrade - Apple
If your Mac is running OS X Mavericks 10.9 or later, you can upgrade directly to macOS Big Sur. You’ll need the following:
- OS X 10.9 or later
- 4GB of memory
- 35.5GB available storage on macOS Sierra or later*
- Some features require an Apple ID; terms apply.
- Some features require a compatible internet service provider; fees may apply.
You should not need to upgrade to an intermediate version.