Paperclip migration error in rails

What the error says is that you must specify in your migrations the version of Rails you are working with; you do this by adding the version in brackets directly in the class you inherit from ActiveRecord::Migration; for example, the error message shows how to do it for Rails 4.2:

class AddAttachmentAvatarToProductossnack < ActiveRecord::Migration[4.2]
  ...
end

In your case, that you are using the version 5.1.1then you would do the following:

class AddAttachmentAvatarToProductossnack < ActiveRecord::Migration[5.1]
  ...
end

The file you must edit is: db/migrate/20170822195101_add_attachment_avatar_to_productossnack.rb ; which was generated when you ran the command:

$ rails generate paperclip productosnack avatar

I can't comment yet but helping you with your comment in the previous question would be in this way the way to build the migration.

class AddAttachmentAvatarToProductossnack < ActiveRecord::Migration[5.1]
  def up
    add_attachment :productossnack, :avatar
  end

  def down
    remove_attachment :productossnack, :avatar
  end
end

Command:

rails g migration AddAttachmentAvatarToProductossnack