Using Memberstack 2.0? This issue has been fixed. Please post a question here if you have a related issue.
Here are a few solutions to help you.
Signup page URL → Remove any password protection and confirm that your site is published and the slug is correct (including folders!). Video
Email and/or password fields → Video
data-ms-signup is in the wrong place → Video
Add a testing domain → Go to your settings page and add a Testing domain.
Missing credentials → Please confirm that your Stripe account is connected and working.