I think I screwed up!

[vicrock]

I'm prepping a 2019 MBP for sale - followed the instructions on the Apple site, all the steps - but since it was running Catalina, I decided to download and install Monterey - and now I'm getting the dreaded ??

I have an external drive with both a CCC and time machine backup - on separate partitions, but when I try to boot from it, I'm getting a message that says the security preferences don't allow an external drive boot.

Any way to get out of this pickle??