Thank you, I accessed your website and tried to install the APP demo, I see the Internal Server Error 500 on my browser console:
This shows there is an error on the server but we do not have access to be able to see the actual error. Please kindly enable the WordPress debug mode to be able to see what is the actual cause.
One possible reason is what we already mentioned in the demo article and that is the PHP Memory Limit. To be on the safe side please kindly Increase the PHP Memory Limit to 512MB.
Also, contact your hosting service provider and ask them to increase the ** PHP Max Script Execute Time** to 300. It is 30 for you. I installed the WP ServerInfo plugin and you can see those options I talked about by going to Dashboard > WP-ServerInfo menu.