If you are not offered your disk for the install, you may need to erase your drive, then install.
You can use macOS Recovery, your computer's built-in recovery system, to reinstall the Mac operating system. Reinstalling macOS doesn't remove your personal data.
support.apple.com
That's the weird part. It's offered—the disk shows up, and it isn't greyed out—but once I select it, I get the error.
(Why do you need to reinstall? Just routine maintenance? Or, have you had problems with the drive?)
I've had a bunch of weird problems with Big Sur. The biggest is that, no matter how many times I delete/reindex, remove from spotlight privacy and put back, my Spotlight index gets corrupted/erased, so I can't search for anything on my drives. After a regular reboot, the machine starts to index again, and that process finishes, and it works for awhile. Then, after roughly an hour or so (I can't pin it to any particular trigger, or any length of time, it happens even if I don't touch my machine) the index erases. Then, when I click on "Recents" in the finder, it's blank. Searches in mail don't find anything. Even trying to search for applications to launch from spotlight doesn't work. I tried troubleshooting that behavior on its own, but nothing I found worked.
The other bigger thing that'll happen is I'll boot into a user account, and it'll act like it's the first time it's
ever booted into that account. It'll ask me to set preferences for apps that launch on login, it'll ask me to reauthorize apps that I've already been using and are already authorized in the "Privacy" tab of Security & Privacy in System Preferences.
There are other things, but those two alone make me want to go to the reinstall option, after trying to troubleshoot the problems piecemeal.
I think the difference here is Big Sur. The error that you get is pointing to some problem in your system, and it is not the system on your SSD, but the system that your Mac actually boots from - the system snapshot (Your boot volume is securely locked, and you boot to that snapshot. Something has affected that snapshot, so a normal reload/reinstall can't work. Your way out is to erase/install.
Thanks for the additional context! If it comes to that (erase + reinstall), it comes to that, but I'm hoping something else might work first.
OP:
PRINT THIS OUT and follow the steps exactly.
1. Power down, all the way off.
2. Boot to INTERNET recovery
Command-OPTION-R at boot
(yes, you may have already done this, but there are "additional steps")
3. When you get to the internet utilities, open Disk Utility.
4. VERY IMPORTANT STEP:
Go to the "view" menu and choose "show all devices".
5. On the left, locate and click on "the top line" that represents the physical hard drive inside the Mini
6. ERASE IT using "APFS with GUID partition format".
Then quit disk utility.
7. Open the OS installer and try again. The Mini will restart a few times, and the display may go dark for several minutes with no indications of anything happening. Be patient. Just "let it be" for a while.
8. Does this work?
You should get an OS install and then get the initial startup screen "choose your language"...
THANK YOU! This is a method I haven't seen or tried. I'll make sure my backups are in order, then try it! Hoping for the best.?