My mac doesn’t have an administrator acount and now cant do changes or update my mac

The "mount -uw /" command that frequently appears in these kinds of tutorials won't work in macOS Big Sur or later. This is because the root volume is a sealed snapshot, and cannot be modified or mounted as read-write.

In macOS Catalina, that command will also fail, because System Integrity Protection blocks the system volume from being mounted as read-write.

To delete the .AppleSetupDone file in macOS Big Sur via Single User Mode, follow these steps to mount the Data volume (which contains the file in question, and can be modified). Note that this probably won't work in macOS Catalina.

1. Start up in Single User Mode. On supported Intel-based Macs, hold down Command-S as you power on or restart your Mac. Release the keys when white text appears on a black screen. (It's technically possible to get Single User Mode on other Intel-based and/or Apple silicon Macs, but that requires setting an NVRAM variable and possibly downgrading the boot security.)
2. If you're prompted to log in first, do so to continue.
3. When the text stops moving, the command line should appear (sh-3.2). If it doesn't, press the Enter/Return key once.
4. Type the following commands in order, exactly as shown:
   1. Mount the Preboot, VM and Update volumes: mount -P 1
   2. If you're running on an Apple silicon Mac and FileVault is enabled, unlock the Data volume: apfs_unlockfv
   3. If you're running on an Apple silicon Mac, start Data Protection services: /usr/libexec/init_data_protection
   4. Mount the Data volume: mount -P 2
   5. Now that the Data volume is mounted, delete the setup file: rm /var/db/.AppleSetupDone
   6. (Nicely) Restart your Mac: launchctl reboot
5. Your Mac should now restart. If prompted to log in, do so to continue.
6. The Setup Assistant will now appear, and you'll be able to make a new admin account, which you can use to restore admin privileges to your regular account.