It has been explained in this forums about 1827872361 times now.
Here goes one more time. When the blowout rule comes into effect, your coach settings have no meaning. He will try to protect your starters from injury. So only way to counter this is: setting your player starter, sub, reserve of the needed spot and have no more than 9 players listed for the game. So when the blowout rule comes into effect and you have set the positions for your subs, there is noone left for your trainee spot.
If you want to train two spots, you can list 8 players and so on.