Nginx sites-enabled, sites-available: Cannot create soft-link between config files in Ubuntu 12.04

You need to start by understanding that the target of a symlink is a pathname. And it can be absolute or relative to the directory which contains the symlink

Assuming you have foo.conf in sites-available

Try

cd sites-enabled
sudo ln -s ../sites-available/foo.conf .
ls -l

Now you will have a symlink in sites-enabled called foo.conf which has a target ../sites-available/foo.conf

Just to be clear, the normal configuration for Apache is that the config files for potential sites live in sites-available and the symlinks for the enabled sites live in sites-enabled, pointing at targets in sites-available. That doesn't quite seem to be the case the way you describe your setup, but that is not your primary problem.

If you want a symlink to ALWAYS point at the same file, regardless of the where the symlink is located, then the target should be the full path.

ln -s /etc/apache2/sites-available/foo.conf mysimlink-whatever.conf

Here is (line 1 of) the output of my ls -l /etc/apache2/sites-enabled:

lrwxrwxrwx 1 root root  26 Jun 24 21:06 000-default -> ../sites-available/default

See how the target of the symlink is relative to the directory that contains the symlink (it starts with ".." meaning go up one directory).

Hardlinks are totally different because the target of a hardlink is not a directory entry but a filing system Inode.

My site configuration file is example.conf in sites-available folder So you can create a symbolic link as

ln -s /etc/nginx/sites-available/example.conf /etc/nginx/sites-enabled/