Single User Mode

Single User Mode is command-s on startup but it doesn't work with newer Macs.

Start up your Mac in single-user mode or verbose mode - Apple Support

Try Verbose mode (command-v):

Then you might try following the steps here:

https://apple.stackexchange.com/questions/164331/i-dont-have-administrator-accou nt-on-my-mac

Can you run a quick test for me? Boot normally,  > System Preferences > Users & Groups. How many users are listed in the left column of the display? Yours, obviously. Any others? Also, now click the padlock in the lower left, and enter your password. Does the padlock shown unlock? If so, you're using an administrative user.

If you have no administrative user shown there and if this system is administered by somebody else, then you'll want to chat with them. If this isn't administered by somebody else, you'll need to create a new administrative user and log into that.

So you've probably tried all the above and it hasn't worked (and with the Mac 2018 model caveat mentioned above), so you're now trying a sequence such as the following? If so, where does this sequence fail, and what diagnostics or messages are you receiving if any?

https://www.lifewire.com/create-new-admin-account-for-login-issues-2259976

If you've tried all that and if there's still have no admin account and if your attempts at creating the account using single-user have all failed, then I'd create a backup or two on external storage, wipe the disk, reinstall macOS with a new user, and migrate in the data and the applications from the backup.

Is something preventing you from booting up or is this another issue you are trying to address with Single User Mode?

so when I logged in, it also started up normally

The system should not allow creation of accounts without a single admin account present somewhere on the computer.

Reset your Mac login password - Apple Support

You can not enable a root user account without an administrative account.

How to enable the root user on your Mac or change your root password - Apple Support