If you're using Cloudflare's Flexible SSL then you can change your Site URLs to be HTTPS, a safe way to do this is adding this code to your wp-config.php
define('SP_REQUEST_URL', ($_SERVER['HTTPS'] ? 'https://' : 'http://') . $_SERVER['HTTP_HOST']);
define('WP_SITEURL', SP_REQUEST_URL);
define('WP_HOME', SP_REQUEST_URL);
It will use the requested protocol as the site URL, so in Cloudflare you can just create a page rule to send all HTTP traffic to HTTPS, that means the Site and Home URL will always be HTTPS (since you can now use HTTPS).
Your Yoast SEO or Other Plugin sitemap will then be in HTTPS. Be sure to turn on the other Cloudflare SSL settings such as automatic rewrites and also use the official Cloudflare plugin.
Answer : https://github.com/Yoast/wordpress-seo/issues/3877#issuecomment-347184510
Delete your sitemap from google webmaster console and resubmit again new one.
A few other solutions :
Change your seo plugin to another, or delete line if using google sitemap generator:
if(!headers_sent()) header('X-Robots-Tag: noindex', true, 200);
in /wp-content/plugins/google-sitemap-generator/sitemap-core.php
Attention This might cause another problem please backup your original files.