xml site map error

Ask your developer to check the PHP output for your page. I have had this error several times due to un-closed PHP statement.

dude i am simple blogger. not a developer can you give me some advice.

when i change theme to twenty sixteen theme its working but after changing to my original theme xml sitemap its not working

If it works with one theme and not another, the issue is the theme causing the problem; so good to reach out to them about a fix as they may have some function not set properly breaking the sitemap, or they may be setting some kind of redirect to a custom “site map” post page index.

You can try this:
a) resave permalinks
b) clear transients using wp-optimize

However, it’s very telling that if the sitemap works using default theme, and then you switch on another theme and it breaks Yoast — most likely conflict with something in that theme !