Hello Anthony–welcome to the forums! Sorry for the issue here, but let’s see if we can help figure out what happening together.
It seems you’ve already narrowed it down to a theme conflict. If the featured image functionality works in a default WordPress theme but not in the theme you are using, then that typically means that there is something in your theme that is preventing it from functioning properly. I would suggest connecting with the theme developer in this case to see if there are any processes blocking the featured image from functioning.
Sorry I don’t have more of a concrete solution for you, but let’s start with this and hopefully the theme author can help shed some light on what’s happening.
Cheers!
Geoff