My best guess is that the URL you submitted in the Google Search console was not .XML page/URL but HTML page/URL. Similar to what’s reported/suggested here.
As I can see your sitemap is accessible (looks like you have Yoast plugin installed) and you could just add /sitemap_index.xml after your WordPress domain URL (e.g.: https://yourwpdomainanme/sitemap_index.xml) and you might want to use this URL format to submit in the Google Search console. This is not something that we (Kinsta) can control though.
Are you facing the same problem in your WordPress site with the SiteMap error issue as reported in this post/thread (is there any specific error message you got)?
Have you tried the suggestion I shared above?