Change in user Name without creating a second user
I changed the name of my user without creating a primary admin. Now my system has refused to boot after I restart it. It has been like that for up to 24hours
MacBook Pro 15″
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I understand your Mac will not boot subsequent to changing an Admin account user name. If you have a Time Machine backup, now is the time to use it: Restore your Mac from a backup - Apple Support.
If you have no TM backup then please read and follow all these steps, from start to finish, including everything, all the way through to and including the end: If your Mac starts up to an Apple logo or progress bar - Apple Support.
Changing the name of a macOS User Account requires this specific procedure: Change the name of your macOS user account and home folder - Apple Support. The boxed note is relevant:
"Doing this incorrectly could damage your account and prevent you from logging in with that account. Make sure that you have a backup before continuing. ..."