Don't know what you are triying to do but you have to instanciate your Map
class TeamSelection
{
var player:Player;
// i don't know why you don't stock your players directly..
var playerInfo : Map<String,String>= new Map();// has to be instanciated
public function new() {
//create your player
player= new Player();
player.id=1;
player.name="bob";
}
public function changeField()
{
//make use of string interpolation
playerInfo.set('info_${player.id}',player.name);
}
}
class Player{
public var id:Int;
public var name:String;
public function new(){}
}