Trying to install the latest-High sierra, it says I need to log on as an administrator??

Do you only have one user account on this Mac system (is it a MacBook Air)? If you open the Users & Groups pane in System Preferences, does it show the account to be an Admin or Standard type account (is the checkbox checked or unchecked for the "Allow user to administer this computer" item?

There should always be at least one admin type account, and that is the default when you first setup the computer.

Good luck...

Rather than reply to your original question, did you mean to reply to one of the other people responding? It helps to figure out who you are asking questions of, rather than adding further explanation to the original question.

In any event, your comment about finding the system administator under settings is not clear to me ("settings" refers to exactly what app?) Perhaps you have your only account set to not require a login (look in Users & Groups and is the "Automatic Login" option set to Off or to a specific user? How many accounts does the Users & Groups Systems Preferences show you have? If just the single user listed, does it show as an "Admin" account or as a "Standard" account (listed right under the user names along the left side of the pane)?

Good luck...

Any change to the Operating System requires an Administrator, go to: System Preferences/Users&Groups, you should be listed as "Administrator", if not click on your name and change the setting.

You must mean that "Automatic login" is set to off, which means you must supply a username and password when you first start up the system (using either "List of users" or "Name and password" style depending on which of those options is selected).

That is the administrator of the system and that is the name (with the password) that it wants (not your iCloud user/password). You can supply either the long form or the short form of the username along with it's password - you can see both of these if you right-click on the user name in "Users & Groups" and select "Advanced Options..." (use control-click if you don't have a mouse with a left & right button). The "short name" is shown in the "Account name" field and the "long name" is shown in the "Full Name" field.

So, to summarize, the user that you normally login to the system with is the "administrator" and you must be giving it a password, so that is the admin password also.

Does that help clear up the question?

Good luck...

Uh . . .? Then log in as administrator. If you don't have the system password you may not be authorized to make such changes.