Equivalent routes for link_to "Subscribe", subscriptions_path,:method => :post
is
POST /subscriptions(.:format) subscriptions#create
So, you have to pass the paper_id
explicitly. Have a look at link_to.
Update your link to <td><%= link_to "Subscribe", subscriptions_path(paper_id: paper.id),:method => :post %></td>
And Instead of
@subscription = current_user.subscriptions.build(subscription_params)
its should be (as paper_id
is not under subscription namespace, you can access it directly)
@subscription = current_user.subscriptions.build({paper_id: params[:paper_id]})