Where did my Troop Tactics in my Inventory go? 
Once you have built a Stronghold, you can go there to activate Troop Tactics to be used in battle. 
Why can’t I donate [this] Troop Tactic to warbases?
Some units and armies aren’t designed to be used on defense, and therefore cannot be donated. These units include:

Supply Carts
All Fighters, Bombers, and Transports
Expedition Armies that contain any of the above troops
Special Deal Armies
Tactical Helicopters