Boot Camp Cannot Partition for Windows
I was trying to install Boot Camp using macOS Big Sur. For some reason, Beta 1 installed some Snapshots on the Macintosh HD volume that prevented the disk from being partitioned. I did eventually get it working, but the solution did not come from me. Credit goes to user tomadimitrie on the Mac Rumors forums who found the solution.

First I booted into macOS recovery mode, and made sure that Macintosh HD was mounted in Disk Utility.
Then I opened Terminal from Utilities &gt; Terminal.
Then I needed to find out the id of the Macintosh HD drive by using the command diskutil list, and finding the IDENTIFIER of the Volume called Macintosh HD. This ID may be different in recovery mode than the one in regular macOS, so make sure you run the command in the recovery mode terminal. The IDENTIFIER looks something like this: disk1s1.
Then run the command diskutil apfs listSnapshots &lt;Macintosh HD's disk id&gt; to list all the snapshots, all which start with com.apple.os.update-.
Then delete each Snapshot by using the command diskutil apfs deleteSnapshot &lt;disk id&gt; -uuid &lt;snapshot uuid from the previous command&gt;. The last one should return an insufficient permissions, but that is fine.
Reboot into macOS.

Only do this at your own risk.
Again, all credit goes to user tomadimitrie on the Mac Rumors forums, and here is the original post to the solution: https://forums.macrumors.com/threads/bootcamp-not-working-deleting-com-apple-os-update-snapshots-using-tmutil.2243872/post-28654311