Hello. I'm using a service called GameServerKings to run my rust server. There I can upload any mod/plugin and edit it from there. I'm trying to follow this video: https://www.youtube.com/watch?v=JzNPoY8CH4k&ab_channel=srtbull 

When following I've noticed that I can't edit or open the main file. Can anyone help me?

Hi.

You'll have to download the JSON and edit it with Notepad ++ to re-upload it when you're done.
Don't forget to validate your JSON on https://jsonlint.com/