My response here addresses this question.
The certificate in SAML metadata is base64 encoded but not in the PEM format that the dashboard requires for the certificate upload. I have a link in the other post to a tool that will format the certificate appropriately. I also mention which URL is usually the correct one. HTTP-Redirect in most cases. Some IdPs may expect the SAML request as a POST, and then you would use the HTTP-POST binding URL. This is often required if the IdP requires signed authentication requests.