[PvK]

If it's single-player, move the transport next to the planet in Tactical combat, and then pull up the options list and land troops on the planet.

If it's a game without Tactical combat, such as PBW, then the ship class for the transport needs to have a ship Strategy set to one with Drop Troops (If Carrying) as the Primary Movement orders.

If the transport is part of a fleet, the fleet should also have such orders. (There may be ways to avoid that, but I haven't tested them in a while.) However the fleet can have "Break Formation" set to True for ships.

Finally, if the planet has weapon platforms on it, they must be destroyed before the Tactical AI will try to land troops there.

PvK