Error in changing the front-end department from website to application

i am a new member and i start by building a e-commerce website in order to gain experience. My steps had been followed the video of personally making an e-commerce website, but I have some troubles when changing from website layout to application one. I used the extension “Live Server” and when i finished the code “@media” and changed it into application layout, it didn’t change anything. The difference between my project and the one in the tutorial video is the error in console: “Could not establish connection. Receiving end does not exist.”. It made me unable to do the next step, and i couldn’t understand the problems.

I hope that someone will help me solve this problem and give me some clear explanation in order to improve my knowledge 🙁
Thanks very much.