I have asked my web hosting to install magento 2.1 on my site.
I purchased a theme.
I am following the manual where it says I must upload the app, lib and pub folder to my magento root. The manual does not say i must replace existing files but I did.
But when I start my site, I get
is currently unable to handle this request.
If you're getting an error 500 you'll probably find some information into your webserver error log. You should check that log and then you can search the solution based on the error description.