There are two possible reasons that you are not able to launch rockets in battle. The first is that your Launch Pad is damaged; when this is the case, you will still see the "Launch Rocket" tab on the left side of the screen while in battle but will not be able to launch the rocket.
The second reason is that rockets are not available for use in base defense; while defending your base, you will not see the "Launch Rocket" tab in battle. While it would be useful in some cases to be able to use rockets for base defense, this is currently not possible.
Rockets also cannot be purchased (with gold or resources) and used while in combat. You can purchase the rocket, but it will not be available for use until your next battle.