It’s simple. Writing a screenplay is telling a story about an interesting character, who wants something badly, and is having trouble getting it.
Of course, it’s important to accomplish maximum emotional impact and audience connection while reaching a satisfactory ending, not necessarily a happy one. But the three essential elements of creating a great story are:
(1) CHARACTER. Your story must be about an interesting somebody with whom we have some empathy. It helps if the character is also likeable and sympathetic, but empathy is key for the audience to identify.
(2) WANT. This somebody wants something very badly, and it should be a matter of life and death. The stakes are high; there is no other option but to go forward.
(3) CONFLICT. Obstacles create roadblocks, making this goal incredibly difficult, which forces the character to find alternative ways to overcome each conflict while making it still possible to achieve the goal.