Hi @John128 
and welcome to Kinsta Community!
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.
Best regards,
Agus