I wasn't sure whether or not to put this in Bugs & Issues or Suggestions but technically its not an issue, merely a small suggestions to increase the efficiency and completion rates of missions.
The mission system acknowledges when you have completed each task in a mission with an arrow through it.
What is the reason for needing to click 'Complete the Mission'.
In my instance, I flew to SA to complete a mission but by the time I land, the time will have ran out and the mission will be failed because I wasn't able to click a button saying 'Complete the Mission', it seems very basic and considering it already monitors your progress seems a rather easy update to implement which saves frustration.
Additionally, possibly a progress bar or circle which will add a 'bar' as you complete each mini task within a mission would look graphically pleasing aswell.
Any additional suggestions, please feel free to add in the comments for moderators to see.