this is what I did to resolve because I had a folder Rosetta sitting in my /Library/Updates folder which seemed to be hindering things, I also couldn't remove SIP to remove the folder and after looking a several different solutions came up with these steps to resolve:
1) removed appleID from the system , system settings/select ID, remove account
2) booted into safe mode, opened terminal and disabled SIP using: csrutil disable
3) rebooted into safe mode and cleared the folders (in my case Rosetta) from 'VloumeName/Library/Updates' leaving just the plist files in there in my case command was sudo rm -R '/Volumes/Macintosh HD/Library/Updates/Rosetta'
4) rebooted again into safe mode and requested update via terminal sudo softwareupdate -i -a -R
5) system rebooted a couple of times
6) logged into new 15.1 beta 4 (this will vary depending on your upgrade track)
7) rebooted again into safe mode, then turned SIP back on using csrutil enable in terminal
8) rebooted normally and added my iCloud/appleID back to system
hope this helps you or someone else
cheers