WordPress SEO by Yoast Error

HI

I have installed WordPress SEO by Yoast. When I click the XML sitemap, it says’That page can’t be found.” I have uncheck the box(Check this box to enable XML sitemap functionality) and clicked save changes. After a couple of minutes, check the box and again save the changes & I clicked XML sitemap(You can find your XML Sitemap here). Then it says that page can’t be found.

But yesterday, when I clicked again that button, suddenly sitemap page showed up and I add to the webmaster tools but there were errors on post sitemap page. Please I need to fix this errors asap. As you know this is very urgent. Firstly, tell me how can I show the sitemap page ?

Thank you