"Path must be document-level" in firestore rules playground. Why?

I’m learning how to set up the rules for Firestore.

I want to simulate a request of a collection, but it shows an error ("Path must be document-level") and doesn’t let me run the request:

Why is it so? It is a very common thing to request a collection of documents from their API, so why can I not simulate it in the playground?

Here are the rules I’m testing. They don’t exactly work, but that’s a different question:

rules_version = '2';

service cloud.firestore {
  match /databases/{database}/documents {
    match /users/{userId}/todos {
      allow read, write: if request.auth.uid == userId;
    }
  }
}

>Solution :

Even if one can fetch an entire Collection, a match statement in Security Rules must specify a document path, as explained in the doc:

All match statements should point to documents, not collections. A
match statement can point to a specific document, as in match
/cities/SF or use wildcards to point to any document in the specified
path, as in match /cities/{city}.

So, in the simulator, you must specify a document path and therefore check your rule for a specific document.

And don’t forget another important point which is somehow linked to the above: Rules are not filters