While Developing IOS App that writes / reads from Firebase, I have found that it was not writing any data in Firebase or can not read data from Firebase, when I saw console, I found a error like “[Firebase/Firestore][I-FST000001] WriteStream (7fea67f13818) Stream error: ‘Permission denied: Missing or insufficient permissions.'”

So, then I have gone to my firebase account, from left side clicked “database”, then clicked the “Rules” tab,
then found:

rules_version = ‘2’;
service cloud.firestore {
match /databases/{database}/documents {
match /{document=**} {
allow read, write: if false;
}
}
}

I replaced it with

rules_version = ‘2’;
service cloud.firestore {
match /databases/{database}/documents {
match /{document=**} {
allow read, write: if true;
}
}
}

Bingo, it solved the problem.
Below is the Screenshot: