I cannot download Macos Big Sur on my Macbook Pro, even though I have 14.6MB. Why?

Big Sur—Press release states that you will need 35.5 GB of free storage when upgrading from High Sierra or newer, and 44.5 GB when upgrading from an older versions.

Big Sur macOS 11.0.1

https://support.apple.com/macos

App Store:

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

How to upgrade to macOS Big Sur w/tech specs

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

Tech specs:

• 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)

https://support.apple.com/kb/sp833?locale=en_US&viewlocale=en_US

Since you do not state what exact Mac this is:

If you can’t install macOS Big Sur on certain 13-inch MacBook Pro computers from 2013 and 2014

Because you need to have at least 35.5 GB of free space to upgrade to Big Sur. Going to assume you meant 14.6 GBs and not 14.6 MBs.

macOS - How to Upgrade - Apple

14.6 MB storage? That's effectively nil. Forget about updating anything until you reclaim some space for that Mac to work.

How to free up storage space on your Mac - Apple Support

Just to add, 2013-2014 MBPs are currently blocked.

See: https://mrmacintosh.com/big-sur-11-0-1-20b50-released-to-block-install-for-2013-14-13-mbpros/