Xcode was always a big bundle. Each release was slightly bigger and an user has to remove some software to install the upgrade.

The release of Xcode 12 is way bigger and it requires more than 40 GB of free disk space to install the upgrade.

The question is: How to install Xcode 12 when there is nothing else to delete?

Simply: Go to Launchpad and throw Xcode to Recycle Bin to uninstall the current version of Xcode.

This step will release the space consumed by Xcode 11. The delete process will take several minutes and you can check your disk for free space.

df -h

Once you have sufficient disk space then open App Store, search for Xcode and perform new installation.