There comes a point where you have to put training aside and fight.
The techniques you learned while training might come in helpful, but mostly what combat training comes down to is enough rote/muscle memory to stop you from freezing in battle and to stick with the guy beside you.
Command training trains you to develop situational awareness and command delegation.
The training Ender received focused on strategy, but mostly he didn't learn from Battleschool battles, he showed an ability to adapt and win at any cost, which is something that can't be taught.
Rackham is telling Ender that the time for training is past, now it's time to fight. The only way you can know how to defeat someone is in the actual fight.
"There is no teacher but the enemy" when training your sparring partner is "your enemy" without actual conflict you cannot practice the maneuvers you've studied, however that enemy, your sparring partner isn't trying to destroy you. They'll pull their punches and won't deliver a killing blow when you're down.
The key to his speach is the second statement
No one but the enemy will tell you what the enemy is going to do
No amount of study, no amount of book learning or practice will tell you what the enemy will do until the enemy is in a position where they have that choice to make.
It's also a very stirring and poetic way of saying "You're on your own kid".