There is a bug where if you have 10,000 coins you cannot purchase a new army slot until you have 12,500 coins, despite the fact that armies are labeled at 10,000. The "BUY" button remains grayed out and un-clickable despite me having 11,000 coins but once I reach 12,500 it becomes gold.
I can confirm that it only costs 10,000 coins as I just purchased an army slot but the UI will not allow this purchase with anything under 12,500.
good find, this will be fixed soon