What does the "Wasn't able to reach URL" error message mean?

This means that the MageMail server could not reach the site URL that's configured or that the request was redirected in some way.  Usually there are two ways to resolve

Option 1 - Firewall / Whitelist

Usually this is because it is behind a firewall or perhaps some layer of authentication.  In case you may need to whitelist the MageMail v2 IP address, it's  45.55.38.132.

Option 2 - Enable HTTPS

If your website is redirecting all http requests to https, that can also cause this issue.  In that case you should enable the site-wide HTTPS setting.