MacBook does not accept name / password?

I have a strange problem with my laptop (Macbook):
I have activated automatic logon, so far so good.
But as soon as I want to install a program or similar, he asks me for username and password. This is always rejected, no matter what I enter!
I can even change the password without further ado, but it is always rejected in the queries.

Does anyone have any advice?

Does your account have administration rights?

What's weird about that? Changes to the system require the name and password and password of an admin, not your username.

Yes he has, it is the only user