I created a website using the webflow.io subdomain, installed the Webflow Pages plugin on my WordPress site, and redirected all the pages of my WordPress site to the corresponding Webflow page. The API key worked fine.
However, I wanted to remove the “Made in Webflow” badge, so I created a new subdomain on one of my personal domains using the instructions outlined here: Webflow Badge with Wordpress Plugin
The website published fine on the new subdomain in Webflow, but now the API key is not working in WordPress. It says “Try publishing the website first”.