1
Try this method first.
Click the power button on the Start Menu
Press the Shift key and click Restart.
Your PC will boot into the Windows Recovery Environment.
Go to Troubleshoot - Advanced Options - Startup Settings and click Restart.
Upon restart, press 4 to enter Safe Mode.
Type Administrator as the user on the login screen and leave the password box empty when logging into Windows as an Administrator.
In Windows, click the Start button, then type netplwiz and press Enter.
In the resulting utility, select your user account and click Properties.
Select the 'Group membership' tab and set your account as a member of the administrators group.
Save the settings and restart the PC.
2
If that process is not successful.
Windows should never let this happen, the must always be an administrator account in Windows 11.
If the only account now left on your PC is a standard account, then you are essentially locked out of making any meaningful changes to your PC and while you will find many 'fixes' on the Internet, sadly, none of them will work because it is a catch 22 situation, you need administrator privileges to make those changes.
Honestly, the only thing you can do now to regain administrator control on your PC is to backup your personal files, then clean install Windows 11 by booting form a Windows 11 bootable USB.
Click this link:
https://www.microsoft.com/en-us/software-download/windows11
to download the Media Creation Tool (Click on Download Tool Now), with that you can download the latest Windows 11 ISO (Select Create Installation Media for Another PC), you can create a bootable USB flash drive (min 8GB) using that tool
Then, Boot your PC from the Installation Media you just created to begin installing Windows 11.