Ruby on Rails No route matches :action=>"show"

Hello i'm newbie in Ruby on Rails i've got following error

No route matches {:action=>"show", :controller=>"collections"} missing required keys: [:id]

I think problem is occuring in routes and here is my code

<% @collections.take(7).each do |c| %>
            <div class="col-md-3">
            <%= link_to c do %>
                <div class="well" style="background: 
                        linear-gradient(
                          rgba(1, 0, 0, 0.4), 
                          rgba(1, 0, 0, 0.4)
                        ),
                        url(<%= c.avatar.url %>);
                        background-size: cover;">
                    <h3 class="text-center"><%= c.name %></h3>
                    <h6 class="text-center">30 places</h6>
                </div>
                <% end %>
            </div>
        <% end %>

And here is routes.rb

resources :collections, only: [:index, :show]
  root 'pages#index'

  namespace :admin do
    resources :places
    resources :collections, only: [:new, :create]
  end

UPDATE!

Here is my collection controller within Admin namespace

class Admin::CollectionsController

    before_action :authenticate_user!
    layout 'dashboard'

    def new
        @collection = Collection.new
    end

    def create
        @collection = Collection.new(coll_params)

        if @collection.save
            redirect_to new_admin_collection_path
        else
            render 'new'
        end
    end

    def show
    end

    def update
        if @collection.update(coll_params)
            redirect_to @collection
        else
            render 'edit'
        end
    end

    def destroy
        @collection.destroy
        redirect_to root_path
    end

    private

    def find_coll
        @collection = Collection.find(params[:id])
    end

    def coll_params
        params.require(:collection).permit(:name, :avatar)
    end
 end