Sharepoint - workbench page shows "This site can’t provide a secure connection" on creating SPFx Webpart

Are you using Node.js v8 by any chance? If so, there is a known issue with the http/2 experimental feature. You can disable it by setting the NODE_NO_HTTP2=1 environment variable. More information about the fix is available at https://github.com/SharePoint/sp-dev-docs/issues/1002.