玉免號&force

You have made the following mistakes on the figures:

Mass of spacecraft = 3780 kg (not 3780 N)
Velocity from 1.7 km/s (= 1700 m/s) to 50 m/s
Height from 15 km (= 15,000 m) to 3 km (3,000 m)

How could you know the time of 720 s ?

You have also used a wrong equation:
F - mg = ma
where F is the propulsion force
m is the mass of the spacecraft
g is the acceleration due to gravity on the moon
a is the vertical acceleration of the spacecraft

The most important point is that the spacecraft is NOT travelling in a vertical descending path, but follows a parabolic path. Hence, your calculated result only gives the vertical force component provided by the rocket.

A second point is that the mass of the spacecraft decreases with time because of fuel consumption.