In order for this to work you must be the owner of the domain and be able to change its DNS settings.
It looks like you have not even registered your domain yet.
Hi @Sara, thanks for the post Take a quick peek at this article, it describes the hosting instructions for your custom domain (which as @makloon indicated, must be purchased separately from Webflow and then pointed at the Webflow servers) Webflow branding | Webflow University Cheers, Dave