@Tom: I had the same issue. In fact the error message is stupid or at least misleading. If it instead would simply say: “Please install gp-premium as plugin not as theme” you at GeneratePress would avoid a lot of complaints and frustration.
I agree, unfortunately I don’t have any control over the error messages WordPress displays.
The instructions to install GP Premium are throughout the site though – I wonder if it would help if I added another notice on the payment complete page.