Business Endpoint - create a team folder

I tried to set up the API call using the information in the documentation.

I added this to the header:

"dropbox-api-select-admin": "dbmid:XXXX",

The body contains this:

{
    "shared_folder_id": "YYYY",
    "members": [
        {
            "member": {
                ".tag": "dropbox_id",
                "dropbox_id": "dbmid:XXXX"
            },
            "access_level": "editor"
        }
    ],
    "quiet": true,
    "custom_message": "This is a new message."
}

But even though I am accessing the API with the admin account, I still get the following error: 

{
    "error_summary": "no_permission/...",
    "error": {
        ".tag": "no_permission"
    },
    "user_message": {
        "locale": "en",
        "text": "You don’t have permission to perform this action."
    }
}

When I use the "user" header, I get the following:

{
    "error_summary": "access_error/not_a_member/...",
    "error": {
        ".tag": "access_error",
        "access_error": {
            ".tag": "not_a_member"
        }
    },
    "user_message": {
        "locale": "en",
        "text": "You aren’t a member of this shared folder."
    }
}

May you help me to solve this issue?

Thank you in advance!

Tereza