can a barbarian use knockback with a thrown weapon? It says your previous action needed to be a successful strike. Am i wrong in thinking i can use this at range?
Yes, per RAW, you can.
The requirement in Knockback is for a Strike
Clearly Strikes can definitely be ranged.
However Knockback says You push the foe back 5 feet, with the effects of a successful Shove
That is the effects of a Shove not an actual Shove
So yes I think Knockback with a thrown or ranged weapon does work.
Most GMs will insist that the target of a Shove is within your reach. Though that is not an explicit restriction on the check,that I can see. But the Shove Trait on weapons limits things to within reach, so you can't normally shove things with ranged weapons. Anyway that is different to your question.