Your theme doesn't have an event card.
All themes must have a customized event card. This ensures an event card functions properly when your page is shared on social networks.
To properly create and style your event card, you’ll need to go into the Event Settings; this is done by first clicking on the Settings tab in your event page's toolbar. Once you've opened Event Settings, click on the purple Edit Event Card button. This will bring you to the Event Card Workspace, where you can stylize your event card to make it consistent with your page's aesthetic and theme.
*Note that you have to save your entire page for the event card to populate correctly.