Thank you, yes I purchased the separate license from Envira Gallery (that wasn't my issue).
However, I was able to resolve it, as Envira reached out and it actually had to do with a setting in the configuration tab.
The "Number of Gallery Columns" was set to automatic and I needed to specify the columns in order to get the dropdown for the Envira templates.
This was Envira's response in case anyone ever gets stuck on this topic:
It looks like the 'Automatic' layout is currently selected, which is why you're not seeing the Gallery Themes dropdown under the Configuration tab. https://enviragallery.com/docs/gallery-themes-addon/#envira-gallery-themes-FAQ-themes-dropdown
In order to access the different Gallery Themes, a specific column count must be set for each gallery. After doing so, the Gallery Themes dropdown will appear for you.