post-excerpt not one of the options under screen options

I want to turn the post-excerpt function on but it doesn’t even show as an option under ‘screen options’ for posts. How do I get it to display as a check box under screen options?

I have tried inserting the code…// Change what’s hidden by default… into the function.php but this causes problems. Perhaps I am not inserting the code into the correct part of the file or something or have left something vital out?

Are you on the Edit Post Screen, or the Edit Page screen?

Pages do not support Excerpts by default, although you can enable it.