I was having the same issue and figured out a solution for my situation. I had created the reCaptcha keys using Google’s new reCaptcha Console, which has this verification as part of the onboarding process. You need to create the keys through the link inside Webflow > Site Settings > Forms and then click the “Register for a new key.” link, which takes you to the older v2 reCaptcha Console. Setup a new key there (with correct domains) and add the reCaptcha field to the forms and it should work.