If it doesn't work again, try removing the beta tester profile and re-adding it before downloading again.
*EDIT - Perhaps the better terminology would be, try un-enrolling from the beta test program and re-enrolling.
I did that before trying to download beta 6 for the first time.
I just woke up to the terminal download finally getting there and I get.
Downloading macOS Big Sur Beta 6
downloading: 100.00%
Failed to download & prepare update: Error Domain=SUMacControllerError Code=7723 "[SUMacControllerErrorPreflightPersonalizeFailed=7723] Failed to perform PreflightPersonalize operation: [MobileSoftwareUpdateErrorDomain(MSU):MSU_ERR_PACKAGE_FAILURE(29)]" UserInfo={NSUnderlyingError=0x7fd9ff61e600 {Error Domain=MobileSoftwareUpdateErrorDomain Code=29 "Package signature invalid." UserInfo={NSLocalizedDescription=Package signature invalid.}}, SUMacControllerErrorIndicationsMask=0, NSDebugDescription=[SUMacControllerErrorPreflightPersonalizeFailed=7723] Failed to perform PreflightPersonalize operation: [MobileSoftwareUpdateErrorDomain(MSU):MSU_ERR_PACKAGE_FAILURE(29)], NSLocalizedDescription=Failed to personalize the software update. Please try again.}
sure enough, softwareupdate -ia goes straight to 15%.
whilst no one else on this forum has posted the same error, googling it finds a thread on developer.apple.com
developer.apple.com
where a number of people have the same error.