rss force_feed()?

1) When I add RSS to my web site, I got “use force_feed()” error message.
I am thinking function.php, but where exactly, and what exactly do i add?
And this is when i finally need child theme?
I just started and haven’t used child theme yet.

2) some RSS just shows URL, not some text from an article.
What can I do about it?

Thanks