[Plugin:SEO by Yoast] –sitemap submission problem

I generated the sitemap using seo by yoast plugin , i got the sitemap of my site (sitemap_index.xml) including these other six sitemaps:
/post-sitemap.xml
/page-sitemap.xml
/category-sitemap.xml
/coupon_category-sitemap.xml
/coupon_tag-sitemap.xml
/stores-sitemap.xml

But actually when i went to this url : https://xyz.com/coupon_tag-sitemap.xml,it’s giving 404 error,page not found
Also when i submitted the sitemap to Google webmaster tool,i got the same error.
How could I solve this?Please Help!