Middleman wont execute for some reason

I've been working on a project and I have been stuck because in ruby if I execute 
<img width="488" alt="Screenshot 2019-11-15 at 09 13 41" src="https://user-images.githubusercontent.com/32667781/68927665-882fc480-0788-11ea-9221-ab219991967d.png">
it simply doesn't execute...