Big Sur installation on MacOS Catalina Retina5k 27, late 2015, 8GB -

I have created a bootable drive to install Big Sur but on running the install receive a message, "this operations couldn't be completed, permission denied". I only have read priveleges for Macintosh HD which is presumably causing the problem but can't find anyway to change the permissions, even though it is a home Mac and I have admin priveleges.

I tried to update from the install app downloaded from the app store and the update goes fine until the reboot process part starts....then it just keeps rebooting....

Any ideas? Thanks, Paul