The get() method is asynchronous, so you either need to use then() to get the querySnapshot when the Promise returned by get() is fulfilled, or use async/await. More details on how to deal with asynchronous calls in this SO answer.

With then()

const functions = require('firebase-functions');



// The Firebase Admin SDK to access Firestore.

const admin = require('firebase-admin');

admin.initializeApp();



const db = admin.firestore();



exports.changeIsVisibleFieldAfterDay = functions.pubsub

.schedule("every 2 minutes").onRun((context) => {

  return db.collectionGroup("Posts").where("isVisible", "==", true).get()

  .then(querySnapshot => {

     querySnapshot.forEach((doc) => {

      console.log(doc.data());

     });

     return null;

  })

});

With async/await

const functions = require('firebase-functions');



// The Firebase Admin SDK to access Firestore.

const admin = require('firebase-admin');

admin.initializeApp();



const db = admin.firestore();



exports.changeIsVisibleFieldAfterDay = functions.pubsub

.schedule("every 2 minutes").onRun(async (context) => {

  const querySnapshot = await db.collectionGroup("Posts").where("isVisible", "==", true).get();

  querySnapshot.forEach((doc) => {

     console.log(doc.data());

  });

  return null;

});

Note the return null; at the end. See this doc item for more details on this key point.

Note also that if you want to update several docs within the forEach() loop, you will need to use Promise.all(). Many SO answers cover this case.