A 2kg squirrel slides at constant velocity down a tree. What is the friction force acting on the squirrel?

Is that all the information given in the problem? If so, I'm not sure exactly but you could argue that since the velocity is constant, there's enough friction to fight the force due to gravity. You're right that the acceleration is zero with constant velocity, but since g = -9.8 m/s^2, the friction force would have an acceleration in the positive direction with the same magnitude to cancel out gravity. Possibly try that value with the the equation for force you already used?