Microsoft Graph - Accessing /me or /user/{id}/ endpoints using client_credentials flow - requested user is invalid - oauth

Question

We're using the client_credentials flow to get access for our application to tenants environments. The application has the correct scopes, and we get an access token that is working for other endpoints like /users but when doing a request like the following we get error messages.
GET https://graph.microsoft.com/beta/me/findRooms
{
"error": {
"code": "ErrorInvalidUser",
"message": "The requested user '{userId}#{tenantId}' is invalid.",
"innerError": {
"request-id": "b72d26a3-d0ad-42eb-a3d3-35951cb42b3d",
"date": "2020-01-21T10:21:28"
}
}
}
I understand that there's no "me" when we're just an application, but how do we access these types of endpoints in that case? Do I have to have a user to act as, as well? That seems to me like it defeats the purpose of a daemon like this. Cannot find any clear documentation on this matter. On this page in the docs on the use a token section they even refer to a /me endpoint, which is incorrect in that case.
I've tried requesting the /users/{id}/findRooms endpoint with all different kinds of ID's I can find in the access token - none of them work.
Other people with the same issue, that have yet to resolve it.
1. Feedback area in docs
2. Github issue
Best regards,
Christopher

Answer 1

Using the /users/{user-id} is the only pattern that will work with client credentials. In your case, this should work, so maybe it's an issue with the id you are using.
To make sure I'm not giving you bad information, I just tested this with an app-only token from the client credentials flow. Parsing that token over at https://jwt.ms, I see the roles claim like so:
"roles": [
"User.Read.All"
]
If first did a GET /users?$select=displayname,id, and this user was included in the response:
{
"displayName": "Adele Vance",
"id": "3103c7b9-cfe6-4cd3-a696-f88909b9a609"
}
This is the id to use in your findRooms call. I did GET /users/3103c7b9-cfe6-4cd3-a696-f88909b9a609/findrooms and got the following response:
{
"#odata.context": "https://graph.microsoft.com/beta/$metadata#Collection(microsoft.graph.emailAddress)",
"value": [
{
"name": "Conf Room Adams",
"address": "Adams#M365x330971.onmicrosoft.com"
},
{
"name": "Conf Room Baker",
"address": "Baker#M365x330971.onmicrosoft.com"
},
{
"name": "Conf Room Crystal",
"address": "Crystal#M365x330971.onmicrosoft.com"
},
{
"name": "Conf Room Hood",
"address": "Hood#M365x330971.onmicrosoft.com"
},
{
"name": "Conf Room Rainier",
"address": "Rainier#M365x330971.onmicrosoft.com"
},
{
"name": "Conf Room Stevens",
"address": "Stevens#M365x330971.onmicrosoft.com"
}
]
}