war "There are no enemy lands within this building's attack radius"

[nkshv.]

I have L1 enemy land "Хомяк" near my L20 land, and I have Guard Tower. When I click "Open fire", map opens and I can see enemy's L1 base "Хомяк" within my attack radius circle. First, it shows me "Land of "Хомяк" is at the lowest possible level and cannot be damaged further! Use army to capture this land". After 1 second it shows "There are no enemy lands within this building's attack radius". So how do I capture enemy land using army?

Edited January 18, 2018 by nkshv.
It saves accidentally

[nkshv.]

Seems the problem exists because I don't have barracks on my main base. But there is no warning for not having it and trying to "Open fire"

[Mr. D]

You are correct, you would need barracks and at least one soldier stationed on the base to attack this land using an army, which can be done from "Earth View", context menu (the "three vertical dots"), "Attack Land" option.

Because of the fact that the Guard Tower can't deal anymore damage to that land it shows the "lowest level" warning (informing you that Army is needed to capture further), and because there were no other lands around you're seeing the second message.

In any case, this action doesn't expend any ammunition (supplies), so it's safe to try opening fire even if there are no targets nearby.

There is no mentioning of barracks as Army combat is completely separate from all attacks made by the buildings (Guard Tower, Missile Launcher and Nuclear Silo): you can attack any land using an army at any time with a similar effect (except that attack structures may be more efficient at destroying walls and such ).