This always happen to everyone at least once, even veterans. It has to do with the default phase stops.
Click the little cog button that is on the left of the screen, between the clocks and inside the phase wheel. There you will see that there are 3-4 phases between attacking and dealing damage, declare attackers, declare blockers, deal swiftstrike damage, deal damage. You should add a stop between, I believe, declare attacker and declare blocker. That way when the opponent taps his troops, you can play your Quick troop in reply and then move to the blockers phase and block.