Why can't you float one permanent magnet directly above another permanent magnet indefinitely by turning their north poles toward one another?

Because they are stationary, the two magnets exert no forces on one another and the top magnet falls.

That arrangement is unstable$-$the upper magnet will fall to the side or flip over.

Two north poles attract one another and the two magnets will pull together until they touch.

The repulsive forces between magnets cannot overcome the gravitational forces between them, so the upper magnet cannot float.