This fully depends on what you are creating. Having no knowledge of your project, I would it assume it falls under 2 buckets: Inventing the Wheel or Reinventing the Wheel.
Inventing the Wheel (The Only Scenario where you can do this)
If you are building something that no one has ever experienced before, you would have no background research to rely on and no one can give you a defined answer of whether you are doing it right. In this case, yes you can probably do the planning without direct interaction with users but you still need to conduct your research to get an empathetic understanding of them.
Reinventing the Wheel
If you are building something that is a manipulation of what already exists (90% of cases), you should definitely incorporate your users as early and as often as you can. You are, after all, designing for them.
You do no need to physically speak to customers but that is ideal so you can catch all the 'unspoken' feedback (reading between the lines). You can also gather the information from customers through surveys or even just an open call for feedback.