You choose which player or planeswalker Reckless Brute attacks.

Reckless Brute attacks only if it's able to do so as the declare attackers step begins. If, at that time, it's tapped, it's affected by a spell or ability that says it can't attack, then it doesn't attack. If there's a cost associated with having it attack, you're not forced to pay that cost, so it doesn't attack in that case either.