Katerina says deal 2 damage to an enemy unit. You have have to do this since it does not say may? And in this case if the only enemy unit has deflect. Can yourbefusr to pay deflect to null the ability?

Yes — you can refuse to pay Deflect and the ability will fizzle, but the trigger itself is mandatory.

Katarina, Reckless's ability ("When you play a card from face down, deal 2 to an enemy unit") has no "may," so it must be placed on the Chain as a Pending trigger whenever the condition happens. But when it finalizes, if the only enemy unit has Deflect, choosing it adds a mandatory Power cost — and at the Pay Costs step, you may decline to pay for a triggered ability (Rule 404.2). If you decline, the ability is removed from the Chain, never finalizes, and deals no damage. This isn't countering — the ability simply never happens. If you do pay the Deflect cost, the 2 damage goes through.