can you attack with yasuo legend ability

Yes — but only in one direction: Yasuo's legend ability (Unforgiven: [2], [E]: Move a friendly unit to or from its base) can send a unit from your base to a battlefield, which is how it "attacks." It can't reposition a unit that's already at a battlefield.

When you move a friendly unit from base to a battlefield you don't control, that applies Contested and stages a Showdown there — Combat if enemy units are present — with your moved unit as the attacker. The unit doesn't need to be ready, since the ability isn't a standard move and doesn't cost the unit's exhaustion (FAQ #6151).

Timing limits matter: this is a base-speed activated ability, so it's only usable on your turn, in an Open State during your Main Phase — never during a showdown or on the opponent's turn, and it can only move one unit, so you can't stack it with a normal move to bring two units into the same fight (FAQ #2721).