attachment.php not working

Im new to WordPress and have been having trouble with attachment.php. I’ve searched support and forums but haven’t found any workable answers.
If I do a “link to page” on an image and I have an attachment.php file, the attachment page only works when I’m logged in as administrator. Otherwise, I get a None Found message.

I’ve tried changing some of the php lines to see how it impacts the page (good way to learn), but not successful.
Some answers in the forum suggest the problem is a bad post_parent number in the database, but I’m not familiar with how to change that.

What I’d like to see is a page of code for a basic attachment.php file that works. I’d like to be able to show the image with the basic information about it, with a comments form.
Is there already one that I haven’t found?

Thanks