I had the same problem updating from Xcode 12 to Xcode 12.01 on a 13" MacBook Pro with 250 gb non-upgradable laptop. The system had 31 GB storage after all the steps described here had been taken, apps deleted, Xcode moved to iCloud. Documents, pictures, large files all moved to iCloud. Moving Xcode to iCloud did not free up disk space as one would imagine. Downloading of the Xcode 12.01 pix stopped after 7.09 GB had been downloaded. Searching for Xcode on App store and clicking on the icon said something like "there's not enough space on your disk to install Xcode". A more helpful message would specify how much storage is needed. The folder /Users/Shared had folders named "Relocated items" and "Previously Relocated items" that occupied 39 GB of storage. The read me in that folder explained that these files could not be moved into their original locations because of the upgrade. These are accumulated over successive upgrades and become huge. Deleting the files in /Users/Shared provided a total of 81 GB of free storage. After installing Xcode 12.01 the system has 64.91 GB with apps taking up 16.42 GB.