Operation Could Not Be Completed. Permission denied.

Trying to install Sequoia on an Early 2015 MBA (4GB RAM).

I get right to the part of rebooting, click install Sequoia, accept Ts & Cs, choose hard drive, I get the “20 minutes” estimate for about 5 minutes, some progress on the bar and then a pop up that says “The operation could not be completed. Permission denied.”

I reboot and I’m back into Monterry which seems OK and now I have 15.5 as an OTA? What’s going on here, how do I overcome this?

Thank you