@AndreasDau ...
Are you using a paid Canvas environment where your Canvas URL would look something like:
https://SchoolName.instructure.com --- where SchoolName is the name of your school
Or, are you using the Free For Teacher version of Canvas --- where the URL looks like this:
https://canvas.instructure.com/
If you are using the FFT version of Canvas, there is no "admin" functionality. Canvas administrators can be created in a paid Canvas account. Paid accounts are typically used by schools, places of higher education, and some other businesses to host their online curriculum. As far as I know, you cannot purchase a Canvas environment as a single user.
If you could please provide some clarity about the kind of environment you are using, that would be helpful. The documentation that @robotcars provided in April 2020 is accurate...except that the link doesn't work any more. Here's the working link for you: How do I add an admin to an account?. (The link you had tried in his response probably broken because in the summer of 2020, the Canvas Community changed platforms, and not all links migrated properly. You just happened to stumble on one of those broken links. If that happens again, you can typically use the search box at the top of any page here in the Community to enter in the name of the Guide, and it should give you a current, proper link.)
The other link that he provided could also go to: Canvas LMS - Instructure Community. Click on "READ THE GUIDES" button, and then choose the role.
