Looks like Yoast has gone out of its way to cripple WordPress sitemaps after you install Yoast!!!
After version 5.5 WordPress will create a sitemap for you at this address /wp-sitemap.xml
Seems Yoast does not like this and disabled the core sitemap. (another reason not to use Yoast!)
don’t worry we have you covered.
1. Add the code below to your functions.php file in your theme.
2. Save the permalink structure again.
3. Un-Cache everything.
4. Make sure WordPress is at least version 5.5
add_filter( 'wp_sitemaps_enabled', '__return_true' );