Ruhan must attack the chosen player if able, not a planeswalker controlled by the chosen player.

If, during your declare attackers step, Ruhan is tapped, is affected by a spell or ability that says it can't attack, or hasn't been under your control continuously since your turn began (and doesn't have haste), then it doesn't attack. If there's a cost associated with having Ruhan attack the chosen player, you aren't forced to pay that cost, so it doesn't have to attack that player in that case either.

If Ruhan isn't forced to attack the chosen player for one of the above reasons but can still attack, you may choose to have Ruhan attack another player, attack a planeswalker, or not attack at all.

If your turn has multiple combat phases, Ruhan's ability triggers at the beginning of each of them. Ignore any choices made during previous combat phases.