[Vuejs]-Resolving an Error 400 with Firebase/Firestore and VueJS

It looks like you are passing array as a value to items key. If you use dbMenuRef.add, you might want to add as sub-collections. Instead, you can use .set if you want to have the entire document added (please refer the documentation here. Can you try:

  addOrder() {
      dbMenuRef
      .doc(this.orderID)
      .set(this.newOrder)
  }