I found the command line to install the update and running it right now, hopefully this time, it will go through: sudo sh -c "softwareupdate -ia && reboot" Here is the result of this command: 2020-08-22 15:30:11.288 softwareupdate[542:5594] Creating client/daemon connection: 5F61A821-0A19-452D-A5BF-515CAA3FD079 Downloading macOS Big Sur Beta 5 downloading: 100.00% Failed to download & prepare update: Error Domain=SUMacControllerError Code=7740 "[SUMacControllerErrorPrepareFailed=7740] Failed to perform Prepare operation: [MobileSoftwareUpdateErrorDomain(MSU):MSU_ERR_FILESYSTEM_DOESNT_VERIFY(3)_1_NSPOSIXErrorDomain:92|FullReplacementRequired|]" UserInfo={SUMacControllerErrorIndicationsSummary=|FullReplacementRequired|, SUMacControllerErrorIndicationsMask=2, SUMacControllerErrorIndicationsDescription=-> Full replacement is required [as opposed to incremental patch]., NSUnderlyingError=0x7fbaf8718b70 {Error Domain=MobileSoftwareUpdateErrorDomain Code=3 "'./System/Library/CoreServices/ManagedClient.app/Contents/PlugIns/mcx.profileDomainPlugin/Contents/Resources/ms.lproj/SystemMigration.stringsdict' did not verify and is not on the exception list." UserInfo={target_update=20A5354i, NSLocalizedDescription='./System/Library/CoreServices/ManagedClient.app/Contents/PlugIns/mcx.profileDomainPlugin/Contents/Resources/ms.lproj/SystemMigration.stringsdict' did not verify and is not on the exception list., NSUnderlyingError=0x7fbaf87189e0 {Error Domain=NSPOSIXErrorDomain Code=92 "Illegal byte sequence" UserInfo={NSLocalizedDescription=lstat("/System/Volumes/Update/mnt1/System/Library/CoreServices/ManagedClient.app/Contents/PlugIns/mcx.profileDomainPlugin/Contents/Resources/ms.lproj/SystemMigration.stringsdict") failed. errno: 92 - Illegal byte sequence}}, MSUFullReplacementRecommened=true}}, NSDebugDescription=[SUMacControllerErrorPrepareFailed=7740] Failed to perform Prepare operation: [MobileSoftwareUpdateErrorDomain(MSU):MSU_ERR_FILESYSTEM_DOESNT_VERIFY(3)_1_NSPOSIXErrorDomain:92|FullReplacementRequired|], NSLocalizedDescription=Failed to prepare the software update. Please try again.}

Same issue here, have a 2019 iMac 21"

Same issue here, have a 2019 iMac 21"