Getting content of Team's root folder, can't find good documentation of Dropbox-API-Select-Admin

That "Unexpected select user header" error indicates that you're using an access token for an app that is not allowed to use that feature. That feature is only meant for "Dropbox Business API" apps registered for the "team member file access" permission.

So, for instance, you might be using an access token for a "Dropbox API" app, e.g., with the "full Dropbox" or "app folder" permission. In that case, you don't need to set a 'Dropbox-API-Select-Admin' or 'Dropbox-API-Select-User' header at all. The access token would only be for a single specific account anyway, so you don't need to additionally specify a particular team member ID. (You can still use the "Dropbox-API-Path-Root" header in that case.)

If you did intend to use the "Dropbox Business API" to connect to entire Dropbox Business team(s), in order to access any team member's files via the "member file access" feature, you can register a "Dropbox Business API" app with the "team member file access" permission here. 

You can find the documentation for the "member file access" feature here.

And you can find the documentation for using the "Dropbox-API-Path-Root" header to access different parts of a Dropbox account/team here.