Can opponent play a reaction spell Flash to me using my Viktor Legend ability?

Yes, your opponent can play Flash in response to your Viktor Legend's activated ability.

Viktor (Herald of the Arcane) has an activated ability — "1 energy, exhaust: Play a 1 Might Recruit unit token." Activated abilities go on the chain, creating a Closed State where opponents can play Reaction spells (FAQ #4710, FAQ #6162). Flash has the Reaction keyword, so it can be played in that window.

The sequence works like this: you activate Viktor's ability → it goes on the chain → your opponent can play Flash before it resolves → then Viktor's ability resolves and makes the Recruit token (which itself resolves immediately with no further window).