Startosinstall failed with error code 9

Using MDS3 on a MacBookPro 9,2 that was running Catalina and then froze and would not fully start up.
Trying to reinstall the Mac OS.
The prepopulated target is the apfs - DATA partition, which happens to be FileVault (encrypted).

There is no prompt to access FileVault by choosing User and entering Password (which I had to do to backup the data to external drive)

Error message dialog box I get says:
startosinstall failed with return code 9. Please
look at the terminal output or check /var/log/install.log for more details.
<> <>

I attempted to decrypt the volumes following derFlounder web site instructions. It began decrypting in the background, but after that finished, the disk still said it had FileVault, although Unlocked.

What does error code 9 tell me?
The terminal window says Error: could not find target…

What next?

I believe you can erase the Mac with a Erase option in recovery, then boot to internet recovery to do a restore / MDS workflow.