Admin status lost after username change

Hi All,

I bought a second hand imac a week ago. It's an old 2011 imac, with High Sierra installed.

I tried to change the username of the user (which was "azerty"...) using the user and profile tools, but when I restarted the mac as requested, I noticed that :

1) the username wasn't changed. 2) it's not admin user anymore !

It seems to be a know high sierra bug...

So I now have a mac with no admin account... I can't install software nor create folder/ copy/paste file in the backup disk....

I saw that there is some workaround using Single user mode, but now comes the tricky part... I then realized that the mac is EFI locked.

I contacted the seller, he said he got it couple of years ago from nearby company when they closed business.

What can I do, is there any way I can get an admin account back on this computer?

can you help me ?

Thanks a lot

Flamand