How to add a new column in an existing table in Rails 5?

You indeed forgot the datatype. You can also do it via the console in the future:

rails g migration AddSkills2ToJobs skills2:string

You forgot to add datatype, below is the updated migration.

class ChangeJobsTable < ActiveRecord::Migration[5.0]
  def change
    add_column :jobs, :skills2, :string
  end
end