Hi Arpenoo,
I’m having the same issue too.
I did some research on the Yoast Website and came across this:
A 404 not found error on the sitemap
1) There’s a conflict with another poorly coded plugin. Or you have a custom post type named ‘Video’ in your WordPress install.
2) If it’s your sitemap index, you should follow these steps: Solution 1
3) If it’s your GEO (Local SEO) sitemap, you should follow these steps first: Solution 2
4) If you have a custom post type ‘Video’, add a single line of code to your functions.php: Solution 3
