Pages and Templates ONLY show code

[ Moderator note: moved to How-to and Troubleshooting. ]

I am a beginner but successfully made a very simple WP site a few days ago, now I want to work on a second one using a theme I bought from Envato. Also I bought Visual Composer. After I uploaded the whole theme and visual composer, and a few other plug ins I set up my first page in the dashboard, the front page. Then I wanted to start designing it. However it only displays as code. I checked other pages and then other WP templates, like 2015, 2017, and OptimizerPro (which I’d used to make my first WPsite). Everything is displaying in code only!
I read the forums and searched the net and have deactivated all plugins, cleared the cache, even switched browsers and still everything is shown only in code.