Facebook recently made a number of changes to its payment system in order to begin phasing out Facebook Credits. These can no longer be purchased from Facebook.
In compliance with these changes, KIXEYE games no longer offer the ability to convert Facebook Credits to gold in the gold purchasing menu. However, if you have Facebook Credits that you have not yet used, you will still be able to use them in War Commander.
To do so, you'll need to purchase a specific item in-game; for example, when you want to do instant repairs and have no gold in your account, click the Instant button instead of going to the purchasing menu.
If you have a balance of Facebook Credits, they will then automatically convert your Facebook Credits to gold to cover the cost, and any remaining gold would be deposited to your War Commander account. If you don't have enough Facebook Credits to cover the entire purchase, a menu will pop up prompting you to purchase whatever extra gold is necessary to complete the action.