So, I cannot fly any more.

Where are you trying to fly?  It must be in a restricted area.  Check the no fly zone data for your area.

Hi sir, this is a known issue, we got a temporary for it: go to iOS settings, Display & Brightness, DISPLAY ZOOM, switch to Standard under view.
If you are using iPhone SE or iPhone 5s etc., maybe you can try to take off the drone, hover it at a low altitude and restart the app again. Please do keep an eye on the control when you are doing this.
And our engineers will get it fixed in the coming app's update. Appreciate your kindness.