When you registered, you got 1000 coins free which can be exhausted in refreshing the rankings (you might have noticed your coins decreasing)
I'd like to tell you that coins are only required for refreshing keyword ranking.
Note: Coins are NOT required for creating campaigns.
I'd like to tell you that without coins you can NOT refresh ranking and hence you may buy coins at https://asoeshop.com/buy.php if you want to update ranking.
It does NOT matter whatever your account balance is, It is the coins which is required for refreshing the rankings, NOT account balance.
Please note that Coins are Non-refundable and required only for Keyword Rank Updates / Refresh!