Azure DevOps Server 2020 - Permission Report

Question

I inherited an existing Azure DevOps Server with about 100 users using it. I'm trying to get an overview about the current permissions and I'm not sure, what's the best way to do that.

What I am looking for is a way to see permissions per group and per user.

I stumbled upon a "permission report", which I should be able to get by the Azure DevOps REST API, but somehow that doesn't work. Here's the response I get:

{
    "$id": "1",
    "innerException": null,
    "message": "The method or operation is not implemented.",
    "typeName": "System.NotImplementedException, mscorlib",
    "typeKey": "NotImplementedException",
    "errorCode": 0,
    "eventId": 0
}

Is this "permission report" feature not supported by the Azure DevOps Server (on premise)? Is there another way to get what I'm looking for?