For ads to be available you need a strong internet connection. Also, VPNs or firewalls (ie from a work location) can prevent the connection to our ad providers.

Few things to try:

Turn on Airplane mode and then turn it off to reset your connection
If you are trying to watch an ad immediately after you open the game, try waiting a couple of minutes – it can take time to connect to our ad provider’s servers.
Log out and then back into your Google account or Game Center account
Ensure you have enough free space on your device – ads require downloading videos that can be large in size.
ANDROID: Clear the cache for the game by going to your device Settings -> Apps ->
Money Tycoon City -> Storage
iOS: Reinstall the app, as you will only download the currently used assets. This won’t lose your game.
ANDROID: Try moving the app from your SD card to internal device memory, or vice versa — it should be in the storage location with the freest space.
On a data connection and not Wi-Fi
A different Wi-Fi network
Lastly, hard reboot your device (fully power off and then back on again)
As a last resort, you can reset your Ad ID.

NOTE: Resetting your Ad ID multiple times runs the risk of the ad provider blocking you. Please only try this once or twice.

Reset Ad ID – Android device:

Go to ‘Settings’
Under Personal, tap ‘Google services’
Under Services, tap ‘Ads’
Tap ‘Reset Advertising ID’
Tap ‘OK’
Reset Ad ID – iOS device:

Settings
Privacy
Advertising
Tap ‘Reset Advertising Identifier’