How do I fix feed error?

That depends of what that incomplete path path to... /feed-rss2.php contains. Could be a Plugin, a Theme, a white space in a file, or any syntactic error while modifying or adding something.

It is preferable to omit the PHP closing tag (?>) at the end of the files, in files with pure PHP code, to avoid such probability.

This is a common cause of errors with described error message.

Making a Backup before changes in files is highly recommended to
prevent loosing your content.