Thanks for contacting World Warfare Support.
The most likely reason you are unable to capture a city, village, port or league city is due to having insufficient leadership points. It costs 4 leadership points to capture a city and 1 to capture one village or one port. Leadership can be improved by leveling up your Battle Level.
Capturing a league city requires special access. You need to be a Chief-Commander or Staff Officer he promoted in your battlefield.
Please let us know if you have any other questions.