I had made some articles using the original Administrator.
Then I created a new Admin user and deleted the original Admin user.
Now I get this error in the html source code.

<!-- Sourcerer! Comment: The tag has been removed, because the owner of this article does not pass the security level. -->

Where do I access the security levels to fix this?

In the Sourcerer plugin you can set the security levels per area.
Articles that have been made by or modified by someone with that level or above, may use Sourcerer.
If you have deleted your admin, then that user can't be found. So Sourcerer can't see it was an admin. You can (and should for other reasons too) do an update over your database, setting all the articles by the old admin to the new one (user id)

Ok, thank you.
That is what I did although I had to go through and re-save all the articles with the new admin user instead of altering the database.

Maybe there should be an error notice to the administrator to notify about errors like this besides just in the html output.
I did not find out about this until a site visitor sent some feedback.

Well, it is not very common to delete your admin.
And doing that (deleting a user) wipes out any user rights attached to it. This is also true for other plugins that do things according to user group levels. So I don't think Sourcerer! should check that. It already notifies you through the comments why certain stuff is not in your output.

I had created a new admin and deleted the original one for security reasons. But now I realize I should have just renamed the user name.