It might be worth you posting this on the Yoast SEO plugin support section - https://wordpress.org/support/plugin/wordpress-seo as they maybe able to provide more help.
However it sounds like you have both a Sitemap generating plugin and Yoast SEO plugin installed, or have had in the past?
If so, it would appear they both are trying to generate a sitemap for you, and are conflicting so you can simply switch off the sitemap functionality in yoasts plugin or remove the other sitemap plugin, then go and remove the sitemap.xml file on your server and get which ever plugin you choose to regenerate it.
Hope that makes sense, if you are in any doubt though, worth using that above link and asking the Yoast support peeps :)