Clutch problem. New Build

Look again. That "shaft" or pin protruding from the center of the drive sprocket is called the 'bucking bar'. There should be a ball bearing in the same hole the pin is poking out of. If that ball isn't there the clutch arm will travel loosely and not disengage the clutch.

Was it like this from the start or did you remove the clutch actuator cover and the pin. It's common for the ball to fall out without noticing while the cover was off. The ball goes in ahead of the pin. And, all of those parts need to be lubricated well with a good grade bearing grease before final assembly.

Another thought is that you're just not pushing the clutch arm far enough to feel the resistance of the clutch spring. Many first time builders under estimate the strength of that spring. You'll have trouble pushing the arm inward with your hand when it meets the spring pressure. The arm will rotate loosely outward and even toward the front but it is supposed to rotate counter-clockwise as viewed from the top looking down to disengage the clutch.
Tom