firebase cloud functions iterate over a collection

You need to run get query, not sure what is admin here, but you can do like this:

const citiesRef = db.collection('cities'); // pass your collection name
const snapshot = await citiesRef.where('capital', '==', true).get(); // query collection
if (snapshot.empty) {
  console.log('No matching documents.');
  return;
}  

snapshot.forEach(doc => {
  console.log(doc.id, '=>', doc.data());
});

You can check this link, for more details.

Also, I would suggest you go through this link to correctly set up the cloud function for firestore if there are any issues with it.