The disk "Macintosh HD" can't be unlocked notification

WaltFrench wrote:

How to diagnose why?

i suspect some app or daemon is looking to this disk by name since “Macintosh -HD” is the root name

But I had trouble installing Catalina on my new MBP & the Apple Tech told me to select “Macintosh HD - Data” for where to install. That’s now my locked system disk and my normal files are on “Macintosh HD - Data - Data”

You need to start over by erasing/formatting the Parent drive.

From Internet Recovery or a bootable USBinstaller>DiskUtility>View>Show All Devices

it is here you see the Parent drive to erase/reformat/initialize not the Container or Child Volumes below.

How to reinstall macOS from macOS Recovery - https://support.apple.com/en-us/HT204904

You then restore your user data from your backups.

The advice you got from the Apple person appears to have been faulty because you now have this odd non-standard designation for that volume. Perhaps your system seems "ok" for now, but the pop up is a symptom of a problem. I wouldn't (personally) proceed to upgrade to a new version of the OS (Big Sur) with volumes mis-named (in fact I would not feel comfortable applying interim security and OS updates with that arrangement), I don't know what the Big Sur installer will do when it does not find what it expects.

I think the suggestion to erase/format the drive is the right one and will result in a "clean" system/setup for you. You can guard against the loss of files by making multiple backups, say one (or two) with Time Machine plus another using a different method, like a "cloning" tool (there are several commercial options, such as CCC, SuperDuper, etc.).