mac os Big Sur cannot be installed on HD

Not near enough free space, & I wouldn't attempt with less than 50GB free...

macOS Big Sur - Technical Specifications

General Requirements

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.

Mac Hardware Requirements

For details about your Mac model, click the Apple icon at the top left of your screen and choose About This Mac. These Mac models are compatible with macOS Big Sur:

MacBook (2015 or later)

MacBook Air (2013 or later)

MacBook Pro (Late 2013 or later)

Mac mini (2014 or later)

iMac (2014 or later)

iMac Pro (2017 or later)

Mac Pro (2013 or later)

Learn about compatibility and how to identify your specific Mac model

https://apps.apple.com/us/app/macos-big-sur/id1526878132?ls=1&mt=12

To add to the good reply from BDAqua, in addition to having the drive space available for installation, you'll also want to calculate a buffer of available free space that allows the OS room to operate properly.

A safe minimum amount of available space on any startup drive is generally considered to be 10%-15% of the startup drive's total capacity. So for example, if your startup drive is 500GB total capacity, you should always allow 50GB-75GB to be freely available.

Just food for thought and good computing.