Technically, you only need to submit a URL (Domain Name or Subdomain Name) and a Keyword to start an SEO Project. Even if you haven't built a website yet, the system is still going to run a health check and competitor check with the given data. However, without any live website pages to review, the results will be too broad and won't help you get the most out of the SEO tool.
We recommend that you build your website first and get it online, in order to maximise the SEO tool.
Starting an SEO Project with a website that's already done and online will allow the SEO tool to crawl through its website pages to check for areas that need fixing, like broken links, duplicate meta tags, slow loading pages, and much more. It also uses advanced algorithm that takes the provided Keywords and compares your website against the top 120 competitors online.
Ultimately, you'll want to start an SEO Project only when you already have a website online to yield SEO Actions that are relevant and on point towards reaching your SEO goals.