"First argument in form cannot contain nil or be empty" Trying to upload document using Carrierwave

I am building my first useful Rails application for a teacher to post assignments and students to upload their work (jpeg pdf) on the website using Carrierwave.

tutorial I want to replicate: http://ift.tt/2chnDAM

Error: "First argument in form cannot contain nil or be empty"

Error message: Extracted source (around line #14):

13    <div class = "well">
14       <%= form_for @submits, html: { multipart: true } do |f| %>
15          <%= f.label :name %>
16          <%= f.text_field :name %>
17          <%= f.label :attachment %>

model: submit.rb

class Submit < ActiveRecord::Base
    mount_uploader :attachment, AttachmentUploader # Tells rails to use this uploader for this model.
   validates :name, presence: true # Make sure the owner's name is present.
end

controller: submit_controller.rb

class SubmitController < ApplicationController
 def index
      @submits = Submit.all
 end

   def new
      @submit = Submit.new
   end

   def create
      @submit = Submit.new(submit_params)

      if @submit.save
         redirect_to submits_path, notice: "The assignment #{@submit.name} has been uploaded."
      else
         render "new"
      end

   end

   def destroy
      @submit = Submit.find(params[:id])
      @submit.destroy
      redirect_to submits_path, notice:  "The assignment #{@submit.name} has been deleted."
   end

   private
      def submit_params
      params.require(:submit).permit(:name, :attachment)
      end

end

routes:

Rails.application.routes.draw do

resources :submit, only: [:index, :new, :create, :destroy]

  get 'welcome/index'

  get 'welcome/about'

  root 'welcome#index'
end

schema:

ActiveRecord::Schema.define(version: 20160903040246) do

  create_table "submits", force: :cascade do |t|
    t.string   "name"
    t.string   "attachment"
    t.datetime "created_at", null: false
    t.datetime "updated_at", null: false
  end

end

Thanks in advance to all the wonderful people here who are willing to lend a helping hand