Grant promotion (Action operator)
In a single player game, this function grants a player an automatic promotion to the next rank which the player can obtain. If he is already at the highest rank, this operator has no effect. It takes no arguments.
The grant-promotion and the grant-medal SEXP work only in single player mode so I presume it's disabled there as the wiki entries for medals.tbl and rank.tbl hint different multiplayer treatment, so if you reached the score and nothing happened it is most likely impossible.
You can of course try to fake it, atleast you could mention that the player was promoted/got a medal.