So I installed High Sierra on my Mac Mini and it ask for a password which i never entered

So this is the problem.

I bought a mac mini 2010. Had a hard drive in it (1tb with yosemite)

I took the HD from my old 2006 mac mini (ssd) and swapped them.

Everything worked. Great success!

But my ssd is 10.6.

I attach the yosemite and then startup with it.

the mac mini starts up.

then i say upgrade to sierra (which is available on the 1tb yosemite hd)

so it starts installing high sierra, which takes a very long time! I go sleep and yes, it installed overnight.

my old ssd username was davidgilmour.

so it shows the login screen with davidgilmour and another user David Gilmour.

it asks for a password. I enter nothing because i never used a password on my ssd old mac mini.

the 1tb yosemite also did not have a password.

and nothing happens, it keeps asking for a password!

i can’t login (when i push the enter key) it remains on the login screen!

so what happened? Did it create a password without telling me? How do i fix this?