See above

Have you converted your AP into Star Coins? If not, click on the AP button underneath your HP/MP bar. It will bring up a window where you can do convert it. After that is done, you will have unbound Star Coins in-game which you can do various things with.

I assume you are trying to buy silver in the market?

I'm pretty sure he wants to get star coins using his AP points, Ive noticed its rigged so you can't used freely earned ones though or it hasn't been implemented yet