Custom domain connection requires a paid Site plan in Webflow. If you’re unable to connect your domain, first verify that you have a Site plan rather than a Workspace plan. You can check this in Workspace settings > Billing. If you have the correct plan but it’s assigned to a different site, you can transfer it to the intended site using our plan transfer guide.
Hopefully this helps! If you still need assistance, please reply here so somebody from the community can help.
Did you purchase the CMS plan on the right project?
I can’t see your site hosting settings through the readonly designer link, but you’ll want to go there to ensure you have the Site plan setup correctly.