How do I add this translation?

I am working on a Sharetribe website. I added the following line to my person.rb file:

validates_presence_of :address_line_1, on: :update

But when the error notification message appears, it says translation missing: en.layouts.notifications.[:address_line_1, "can't be blank"]

I've searched online and I can't see how I should add this translation?

FYI, Sharetribe runs on Ruby 2.1.2 and Rails 3.2.21.