Big Sur update from App store "not available"?

I'm trying to update from Catalina 10.15.7 to Big Sur on my early 2013 MacBook Pro. The App Store says my computer is compatible, I have plenty of storage, I'm signed into my Apple ID, I have my permissions set to allow updates from the App Store, and I'm not running any other programs on my computer, but when I click the "Get" button, I just get an error message that says "Update not found. The requested version of macOS is not available" and then my System Update window says my computer is all up to date.

What's going on here?