Sitemap Couldn't Fetch in Google Search Console

I'm having an issue with my website https://gradescalc.com/. When I try to submit my sitemap to Google Search Console, I receive an error message saying "Sitemap could not fetch." I've double-checked the URL, and it seems correct.

Despite the efforts, Google Search Console still can't fetch it. Has anyone else experienced this issue? What steps did you take to resolve it? Any advice or troubleshooting tips would be greatly appreciated!

Thanks in advance!