There are few problems here

1. There should be async-await for both functions
2. Your fieldValue should start from firebase.firestore.FieldValue not firestoreFieldValue
3. Also where clause is used for collection, not doc() so remove that as well. Also I don't think this will update the full collection but do check it and see. (The error you are getting is because of this)

I don't know how you are importing firebase in this application and I don't know how you have declared firestore but mostly firestore variable is declared like this

const firestore = firebase.firestore();

In here firestore is a function, not a property But when you are using it in the FieldValue then it should be like

firebase.firestore.FieldValue.increment(-1),

Notice that firestore here is a property not a function

Your full code should be like this

async function incrementCounter(collref) {
    collref = firestore
      .collection("items")
      .where(selectedItem, "==", selectedItem);

    const newRef = await collref.get();
    for(let i in newRef.docs){
      const doc = newRef.docs[i];
      await doc.update({
       stocks: firebase.firestore.FieldValue.increment(-1),
     }); 
       // You can also batch this 
    }
  }



const handleSubmit = async (e) => {
    e.preventDefault();
    try {
      const userRef = firestore.collection("users").doc(id);
      const ref = await userRef.set(
        {
         ....
          },
        },

        { merge: true }
      );

      console.log(" saved");
      await incrementCounter();
    } catch (err) {
      console.log(err);
    }
  };

The where() method exists on a CollectionReference and not a DocumentReference. You also need to get references to those documents first so first get all the matching documents and then update all of them using Promise.all() or Batch Writes:

function incrementCounter() {
  // not param required  ^^
  const collref = firestore
      .collection("items")
      // .doc() <-- remove this
      .where(selectedItem, "==", selectedItem);
      //      ^^^                 ^^^
      //      doc field           field value
      //      "item"              {selectedItemName}

  collRef.get().then(async (qSnap) => {
    const updates = []
    qSnap.docs.forEach((doc) => {
      updates.push(doc.ref.update({ stocks: firebase.firestore.FieldValue.increment(-1) }))
    })
    await Promise.all(updates)
  })
}

If you are updating less than 500 documents, consider using batch writes to make sure all updates either fail or pass:

collRef.get().then(async (qSnap) => {
  const batch = firestore.batch()

  qSnap.docs.forEach((doc) => {
    batch.update(doc.ref, { stocks: firebase.firestore.FieldValue.increment(-1) })
  })

  await batch.commit()
})

You can read more about batch writes in the documentation