If you are receiving this error message:
It's most likely due to an issue with Icons running in Administrator mode with Steam not running in Administrator mode. Navigate to your icons directory, typically located at C:\Program Files (x86)\Steam\steamapps\common\Icons Combat Arena, and right click on Icons, then selecting properties.
Navigate to the compatibility tab, and make sure "Run this program as administrator" is unchecked, as seen here:
Then, run Icons: Combat Arena through steam and you should now be able to complete your purchase.