The author has not been offering updates for more than 18 months. The template has not yet been tested for Wordpress 5.x. Maybe he is no longer interested in selling this item and it seems he does not care about his former customers.
I am sorry to hear that you are having issues with Mixed theme however theme is working smoothly on recent WordPress versions. If you have issues, please get in touch with us and we will take a look. You probably just need to install recent version of page builder.
Great theme. It's easy to use and has lots of different ways to make it look good. My organization always shy's away from using page builders other than visual composer, but this one is much better than most of the other ones we've tried. There's a few little things with the builder that aren't intuitive, and yet, they aren't a big enough deal to drop the star rating. We're happy with the theme.