If you are trying to login to the iPhone app and you already have an account, but you're getting "This email is not eligible for signup," then click the back button and try it again.
Here's what happened
When first typing in your email address, it might not have been accurate, so Frame.io sees this as a new account. While "creating your new account," the email address was corrected, and now this account can't be created since it already exists.