Email verification links expire after a few days. If you clicked on a verification email link, and got the "Couldn't verify email" error page, you need to re-send the verification email.
To resend the verification email, go to https://formspree.io/account and look in the `Linked Emails` section. Unverified emails will have a yellow dot. Hover over the email you'd like to verify, which will reveal the delete and re-send icons. Click the re-send icon.
Now go to your email client and find the verification email, and click the link.