Building and using nuclear bombs

Players,

To combat a player that uses stacks of 10-15 early in the game, we are three weeks in. Day 21. I decided to use some gold and have researched bombers to level 4.

I have rocket bomb to level 3. Nuclear Fighter to level three and nuclear rocket to one. I also have fortified one airport to three. I still cannot build the rocket. Any ideas? Attachment is below.

Thank you.