Accidentally changed my admin account to standard and now have no admin access for Macbook Pro OS Ventura 13.6.3.

You can create a new admin user account by tricking macOS into running Setup Assistant again. Here is an article with instructions to do this while booted from Recovery Mode:

Restarting Setup Assistant to create Admin user - Apple Community

If you can boot into Single User Mode, then it is possible to modify those instructions I linked, but you would need to understand the command line and how macOS mounts various volumes. The location of the mounted volumes differs between Recovery Mode and Single User Mode. Plus a few extra steps are necessary to make the file system writeable.....the instructions to make the volume writable are printed on screen when booted into Single User Mode. I recommend you follow the linked article by booting into Recovery Mode.

Once you get a new macOS admin user account, you can follow the instructions in the following Apple article to fix the issue with the main user account when you tried to rename it:

Change the name of your macOS user account and home folder - Apple Support

FYI, Apple removed the ability to run Setup Assistant again if the computer is running macOS 14.x Sonoma.

I hope you have frequent and regular backups of your computer and all external media (including the cloud) which contains important & unique data.

The command shown in the first screenshot where you placed it in double quotes looks correct. It either means that the ".AppleSetupDone" file is actually not there as the error mentions, or maybe a later macOS update has put a restriction in place so it is no longer possible to access that file or location. I do know that with macOS Sonoma this is no longer possible and sometimes those types of updates do get passed down to an older OS like Ventura.

Just out of curiosity, try this command which will list all the files in the "db" folder:

ls  -hal  "/Volumes/Macintosh HD - Data/private/var/db"

If Apple did lock down that ".AppleSetupDone" file for security reasons, then you won't be able to do use this method to fix your Mac using this method so you will need to perform a clean install of macOS & restore from a backup. Keep in mind Apple may be considering this a security issue with Ventura since it could allow someone who doesn't know any passwords to take over the system....just like you are trying to do to fix your previous mistake. It is also possible your employer implemented some security lockdowns as well.

Since your command looks correct, you will most likely need to perform a clean install of macOS & restore from a backup. This procedure will destroy all data on the internal SSD. However, if you need to recover data, then you can try creating a new APFS volume and installing macOS onto the new APFS volume (give the new APFS volume a unique name....maybe "Macintosh SSD" so it doesn't conflict with the volume name of the original OS. You should be able to access data within the home user folder of the original OS user accounts while booted to the new OS, although it can be tricky if either OS has multiple users (or had multiple users, or had a user deleted and/or added) since access is determined by UserIDs and not user names.

First, the command you pasted here is incomplete. Not sure if that is just an oversight when copying & pasting it here, but you must be extremely careful since there are no safety nets when using the command line. The " -i " option is a bit of a safety feature in that it gives you a chance to make sure you are deleting the correct item.

Second, is the name of your internal boot volume still the default "Macintosh HD" or did you rename it? If you renamed it at some point, then you must use the correct name for it here. Also, there are actually now two APFS volumes for a macOS boot volume which are tied together behind the scenes.....one is for the macOS system files (default is "Macintosh HD") and the other is the "Data" volume containing your home user folder & data (default is called "Data", or "Macintosh HD - Data" depending on the version of macOS). You need to make sure the "Data" volume is mounted. If you renamed the boot volume at some point, then you could have a mix of "basenames" depending how you renamed it which can complicate things a bit.

If you did rename the internal boot volume at some point and you are unsure of the name or proper path to use in the command you are trying to follow in the linked instructions you are trying to follow, then use the following two commands in the Terminal so we can see what the proper name of the "Data" volume may be now. Do not unmount anything as it may complicate things. Do make sure that the "Data" volume on the internal SSD is mounted though.

diskutil  list  internal

mount

The first command will list the drive layout of the internal drive while the second command will list all mounted volumes and their mount points.