MacBook Pro 2018: macOS reinstall to Mac without existing macOS, admin account & startup disk I'm trying to reinstall macOS to seriously messed up MacBook Pro 2018. Apparently, problems  started initially when Big Sur update failed. Computer's disk has since been erased and all accounts have been lost. The computer shows question mark folder when it's started.
Here's what I've tried so far:

Started Mac to startup manager (press ⌥ in startup). No disks are showing, only option is internet recovery, which fails.
Started Mac in recovery mode and ran First Aid on the disk, which says that the volume appears to be OK. In addition to internal Apple SSD, there is macOS Base System Disk Image.
Also in recovery mode, I've ran command bless --info and bless --getBoot which both return Can't access &quot;efi-boot-device NVRAM variable.
Reset NVRAM and SMC, which had no effect
Tried to install macOS from bootable drive, which fails because external boot is disallowed in Startup Security Utility. I cannot change the setting because there is no administrator account.
Tried to start into single user mode (⌘-S), to remove /var/db/.AppleSetupDone file in order to run Mac setup again and get new admin account. Logging into single user mode fails, apparently because there is no startup disk (based on this: https://discussions.apple.com/thread/8448671).
Tried to set startup volume in recovery mode terminal by running command bless -mount /Volumes/Macintosh\ HD/ -setBoot which returns the following:

Can't load /Volumes/Machintosh HD/usr/standalone/i386/apfs.efi
Could not load apfs.efi data from /Volumes/Machintosh HD/usr/standalone/i386/apfs.efi

Any ideas what I should do? I'm guessing that the first thing I need is to get startup disk working and create an administrator account. However, that seems to be easier said than done.
Agree with X_841.  It sounds like you need to create a USB installer disk and reformat the hard drive or SSD from there.