The article states that's exactly what they're doing. They believe there's a planet nine based on its effect on other objects, and have a theoretical orbital range modeled from those gravitational effects.
The problem is space is big, and space is dark, so its hard to find things that don't generate their own light.
Even if you could predict the exact orbit, you still don't know where on that orbit it is right now. So even best case that's a lot of space to search.
The problem is space is big, and space is dark, so its hard to find things that don't generate their own light.