If the game says there are no ads available, first check if you have an ad blocker, antivirus, or VPN installed on your device. If necessary, disable them or add Zombie Miner to the exception list.
If this didn’t help, there might actually be no ads available for you at the moment. New videos appear automatically as soon as advertisers have ads available to show. Please try checking for ads later: they usually become available after some time.