This issue has been posted here before however the solution previously provided included the download and install of CleanMyMac 3. My solution is so much easier.
Launch Terminal and enter
dscl . list /Users | grep -v '^_'
From the list of users you may see SonosDMS, you need to remove this user for the installer complete. Next command to remove the user is:
sudo /usr/bin/dscl . -delete /Users/SonosDMS
reboot and then launch the Beta 10.3 installer from your Applications folder, it will now install.
