I had to format my Mac and make a new user, the guy who formatted it for me made the acc called "USER" and I changed it to my name from users & groups. It gave me a message that some changes will happen and I agreed anyway but now I have an admin acc that requests a username and password that won't mach any of what I used since the format, and it won't mach the login information (I have no problem logging in), I have an admin acc only and it won't give me authorization, and I can't copy to my partition drive it require authorization that I don't have and a username not changed.
You need an admin user to manage this Mac, so why not create that first:
I like my answer on that post (go figure) and then you can use the new admin credentials to fix your current user short name and home folder.
Lots of other good advice and answers here as well: