If a creature isn't able to attack for any reason (such as being tapped as its controller's declare attackers step begins or being affected by "summoning sickness"), that creature doesn't have to attack. If no creatures a player controls are able to attack, Seeker of Slaanesh's requirement has no effect during that combat. If there's a cost associated with having a creature attack, its controller isn't forced to pay that cost, so it doesn't have to attack in that case either.