Marketing & Sales. A lot of entrepreneurs are "doers of the thing" but at some point, you're forced to take the shift and become a "marketer of the thing".

Think of it this way: You can have the best product on the market, but it doesn't matter if noone knows about it.

Ofcourse, there are so many other important bits I'd love to tell you about - but, start with these and you will see traction, the rest will follow.