Big Sur install error 702

My Big Sur installation gets to about "13 minutes remaining" before throwing an error. The installer log in Console shows the info at the bottom.

The error on "disk3s2" doesn't make sense to me. I'm installing onto "Macintosh HD", which is APFS and, as you can see in the bottom right, is disk1s5:

"Macintosh HD - Data" is disk1s1.

"Shared Services", an image loaded during the installation process, is disk2s2.

I have no other drives connected.

2021-01-31 13:31:30-05 GSRMT-FVFWL05DHV2G osinstallersetupd[1282]: Retrieved package com.apple.pkg.InstallAssistant.macOSBigSur (http://swcdn.apple.com/content/downloads/00/55/001-86606-A_9SF1TL01U7/5duug9lar1gypwunjfl96dza0upa854qgg/InstallAssistant.pkg)
2021-01-31 13:31:30-05 GSRMT-FVFWL05DHV2G osinstallersetupd[1282]: mountDiskImageWithPath: /System/Volumes/Data/macOS Install Data/InstallAssistant.pkg
2021-01-31 13:31:31-05 GSRMT-FVFWL05DHV2G osinstallersetupd[1282]: Disk disk3s2 is not suitable because DMError: -69837
2021-01-31 13:31:31-05 GSRMT-FVFWL05DHV2G osinstallersetupd[1282]: disk3s2 is not convertible to APFS: You may not install to this volume because it can not be converted to Apple File System (APFS).
2021-01-31 13:31:31-05 GSRMT-FVFWL05DHV2G osinstallersetupd[1282]: Disk disk3s2 is not suitable because DMError: -69837
2021-01-31 13:31:31-05 GSRMT-FVFWL05DHV2G osinstallersetupd[1282]: disk3s2 is not convertible to APFS: You may not install to this volume because it can not be converted to Apple File System (APFS).
2021-01-31 13:31:31-05 GSRMT-FVFWL05DHV2G osinstallersetupd[1282]: Couldn't mount dmg! (error code 112)
2021-01-31 13:31:31-05 GSRMT-FVFWL05DHV2G osinstallersetupd[1282]: Failed to find the payload dmg.
2021-01-31 13:31:31-05 GSRMT-FVFWL05DHV2G osinstallersetupd[1282]: Operation queue failed with error: Error Domain=com.apple.OSInstallerSetup.error Code=702 "(null)"