is the coefficient of friction independent of direction?
In this question, since the box is not slipping along the incline, I assume that $ \mu_{current} = \tan 20, \implies P = W \sin 20 $, but I am getting $26.8 N$ which is wrong.
@AjayMishra In real life the friction often depends on the direction, but in questions like this you can assume the friction is the same in all directions unless the question specifically tells you otherwise.
The block is not constrained to move in the horizontal direction. What will happen is that as you increase P the block will start to slip at some angle given by the relative magnitudes of the two forces.