I need 50GB to install MacOS Big Sur Beta. I have 60GB but the update application says I only have 40GB.

I'm trying to update to Big Sur beta. When I try to update, it says that I have 40GB available and I need 10 more GB to install. In reality, I have about 60GB. I have tried rebooting but the number is still stuck. I was downloading the update during a Time Machine Backup, so I'm unsure whether that's the reason my storage space is shown wrongly.

I need 50GB to install MacOS Big Sur Beta. I have 60GB but the update application says I only have 40GB.
 
 
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