the button for save new mission

Show the rest of the screen. You likely don't have a purple dot drawn to show where the takeoff location is or have already canceled out of a previous save. It is hard to tell from that screenshot. 

This is the full screen image. The flight plan is made with the flight points and parameters.

If you are using the LITE version the ability to save missions might not be enabled. Or did you already attempt to save the mission once and it failed? Pre 4.1 will grey that out after a failed map save.