There are a few reasons a URL might return an error. Here are solutions to the most common problems.
Invalid testing domain
Head over to your Site Settings page to fix your testing domain. Note: Memberstack will work on any site. This domain is just for testing purposes.
Memberstack can't see or validate pages that require a password. You'll need to either remove password protection or continue with URL validation.
Your site hasn't been published
This is a great problem, because it's so easy to fix! Just publish your website and try again.