Hi @Cheshire600, welcome to the Forum!
Unfortunately it’s impossible to complete now. That Valor Challenge refers to the monthly Challenge Events (Pirates of Corellia and Knights of Avalon), and requires that all 3 Tiers of each Event are fully completed.
If you’d like to support changes to that for future Paths of Valor, there’s a thread here for commenting and voting on that Valor Challenge: Refine "Complete a Challenge Event" Valor Challenge to Provide Flexibility in Completing Path of Valor