Sorry to hear that you’re having trouble submitting the XML sitemap to Google Search Console.
We have tested the relevant sitemap index using https://httpstatus.io/ while using the Googlebot useragent and indeed it’s returning 403 forbidden HTTP status error.
The issue typically occurs when there’s a conflict with something. Therefore, we recommend you please perform a full conflict and run the relevant test again by using the https://httpstatus.io/ to see whether you can find out the exact cause of the issue.
The fastest way to rule out any conflict, is to deactivate all non-Yoast plugins and switch to a standard theme like Twenty Twenty.
Please test this on your development or staging site, if you have one. If not, we recommend using the Health Check & Troubleshooting plugin. This plugin has a troubleshooting mode, which does not affect normal visitors to your site.
If you’re unfamiliar with checking for conflicts, we’d like to point you to a step-by-step guide that will walk you through the process: How to check for plugin conflicts
