While sitemap generators can be used on free hosting, they are blocked by the system that @wackyblackie mentioned. You have two options.
Ignore it and don’t create a sitemap. They are not really needed for SEO since they only tell the spiders where to look, but they will find all your pages through links on your site anyway.
Use Cloudflare. Unfortunately, you are using a free sub-domain, so this is not possible. If you wish to get a free domain from Freenom, you can follow the guide below