What happens when Tax action from King's Order card played and NO gold coins exist in reserve?
1. Can you create more coins?
------------- OR ----------
2. You can only use the exact number of physical coins present in game. You cannot create extra coins for reserve. Basically the coins are just like army units - you only have what physically exists with the game...
I would probably rule that once the coins are gone, they're gone. The real question is whether it would be possible to hoard coins without being wiped off the board by those that spent them. Given that thought, I think there are probably enough coins provided for this never to be an issue.