[WordPress SEO by Yoast] XML Sitemap not working

Once installed the WordPress SEO by Yoast plugin I can change the sitemap settings, but if I go to the plugin under SEO -> XML Sitemap and press the button for my “XML Sitemap” I once again only get redirected to my start page!

I tried both adding that code in .htacess and changing the init() code of the plugin file class-sitemaps.php as described in this tutorial here: https://www.wpbeginner.com/wp-tutorials/how-to-fix-yoasts-wordpress-seo-sitemap-404-error/ But both did not work….the only change I had that when I changed the init() code in class-sitemaps.php I had one brace stet wrong the first time and the for the first time it would not redirect to my start page but rather show a syntax error under myurl.com/sitemap-index.xml.

I have a WordPress Multsite Installation

Please help!