Are you using the Free Plan?
If so, you are limited to 2 projects (unpublished sites) at a time. If you create a site and host it with Webflow it does not count as one of the 2 projects.
If you purchase an ‘Account Plan’ you can then have more sites (unhosted) at one time. The ‘Lite’ Plan allows 10 projects at a time.
Info here: https://webflow.com/pricing
You have the option of paying monthly (slightly higher price) for the account plan, this may allow you to have more than 2 projects until you’re ready to publish and host.
Hope that helps,
Keiran