Upgrade to Monterey blocked because I didn't "authorize the startup disk"

I had the same exact thing happen to me. On my Mac, I have an "Admin" account and two user accounts. I normally am logged into my user account and only log into my "Admin" profile when I need to do something major to my Mac to which I don't want to botch something on my personal profile. When trying to initiate the update from my user profile, I got that exact message.

What I ended up doing was logging out of my User account and logging into my Admin account and initiating the install from there. Voila, install went without a hitch.

Hello send9,

Thank you for using Apple Support Communities.

We see that you are unable to install macOS Monterey on your Mac mini. You are getting an error message that. you need to select the volume as the startup disk. We would recommend taking a look at the following support article to set your hard drive as the startup disk.

Change your Mac startup disk - Apple Support

Best regards.

You know what, now that you mention that, I didn't notice at all that I wasn't getting prompted for admin authorization when I was trying to do the install while logged into my own profile.