Creating a hard link to an ordinary file returns an error. What could be the reason for this?

A file's hidden attribute does not prevent the creation of a hard link to it, as the link mechanism operates on the file's underlying inode.

A read-only file can still have hard links created to it, as creating a link only adds a directory entry pointing to the existing inode, without modifying the file's content or permissions.

The type of file, such as a shell script, does not impose restrictions on creating hard links to it.

A file can have multiple hard links pointing to it; being an existing hard link does not prevent the creation of additional hard links.

Hard links operate by pointing directly to a file's inode number. Since inode numbers are unique only within a single filesystem, a hard link cannot refer to an inode on a different filesystem, thus attempting to do so will result in an error.