You are probably trying to install Stastic on an existing website - see the section “[How to install Stastic?]”, and when you do, Stastic says that your website is “not a Jekyll site”.
Your website is not a Jekyll site
This means that the website you selected has no _config.yml file in its root folder. This file is all it takes to have a Jekyll website, and it is really easy to add one to a website you have.
See this official documentation to know more about this file and the options it can hold.