There are some reasons below that you might have.
For selling ads
You don't have enough coins in your balance. You need to check you wallet again or your ad. And then deposit more coins to satisfy the ad you have created.
Your balance is smaller than the minimum amount for creating an ad. You need to adjust your min or deposit more.
Your max < min . You need to check and adjust it.
The price you set is higher than other users. You may want to look for your ad in the next pages.
For buying ads
You should check your max, min, or balance to ensure they are appropriate.
Your fiat wallet may not have enough money to buy the amount of coins in your ad. You need to check your wallet and deposit if it is smaller than the amount you have to pay for the coins. .
After you update your ad, you should wait for a few seconds and refresh the page to be able to see your ad.