Thanks for reporting this in!
It seems you might be missing the Default card base image and so the Card Game Combat plugin is unable to render the cards in the Battle Scene. The plugin requires at least one Default card base image as a fallback in case no other Card Art or Card Base images can be found at the plugin parameter specified directories. To resolve this issue, we believe you'll need to choose an image in the system folder for the Default Card Base plugin parameter (in the Demo Projects, this is the Card1.png image in the system folder).
If the issue does not resolve after this image is restored, let us know and we can continue to troubleshoot it with you :)