Is Scrum compatible with a project schedule?
As far as I understand we don't estimate or plan a whole project in Scrum. More over the requirements are allowed to change freely. So does this mean that there can be no schedule for a Scrum project?
In particular, we can never give an estimate of when we complete a Scrum project?
Yes we can, planning in Scrum is done at three different levels: daily planning, iteration planning; release planning.
From team's point of view, the release planning gives us the direction, the iteration planning gives the structure and the daily planning gives the context for the next days.
All the planning allows risks to be identified early and so we're indeed free to revisit and update the release plan as we see fit (at least once per iteration is the ideal).
It's important a good communication so that expectations and requirements are clear so that you keep things on track.