register for blog

I have a blog and forum on my site here I noticed that when someone logs in and makes a comment in the blog, the message ‘file not found’ pops up – this is the url `
https://xaddx.org/wordpress/wp-comments-post.php
‘
I did not see this in the default template folder. Where would I find this file so I can add it to my template?
Also, what other default pages would I need from the default template for my template?

Thanks