I am not able to change the company workflow - what can be the reason?

If you are not able to change the workflow, this means that there are some claims in between two statuses.

As a general rule it is not possible to change unless all claims have the status Open/Rejected/Canceled/Waiting for approval/Approved or Paid/Booked.

So if there are claims with states between Approved and the last state for the flow (Paid or Booked, depending on your current workflow), you need to mark them as Paid or Booked.