Creating Sub-Collection in Document from Flutter app

 final databaseReference = Firestore.instance;
databaseReference.collection('main collection name').document( unique id).collection('string name').document().setData(); // your answer missing **.document()**  before setData

this is the right Syntex

Figured out the problem. I was using the wrong syntax. The correct syntax is

    Firestore.instance.collection('path').document("documentPath").collection('subCollectionPath').setData({});

The key difference here is that the forward slashes have been removed from the paths name.

Here's the solution, if you want to create a subcollection within a Firestore Transaction:

Firestore.instance.runTransaction((Transaction tx) {
    tx.set(Firestore.instance.collection('path').document('documentPath')
        .collection('subCollectionPath').document(), {'TestData', 'Data'});
})

NOTE: You cannot create an empty subcollection, you must create the subcollection with at least one document. If you try to create the subcollection with the data field empty, Firestore will create an empty document.