TypeError: admin.firestore.collection is not a function

It's admin.firestore(), not admin.firestore. It's a function call, not a property. In your code, you're doing it correctly once, then incorrectly twice.

I had same problem for my functions.

exports.getScreams = functions.https.onRequest((req, res) => {
admin.firestore.collection('screams').get()
.then((data) => {
    let screams = [];
    data.forEach((doc) => {
        screams.push(doc.data())
    });
    return res.json(screams);
})
.catch((err) => console.error(err)) })

Then i use

admin.firestore() instead of admin.firestore