Uncle_Theodore is quite correct
ln will not overwrite any existing file or directory, but attempt to create a link one level down.
Code:
cd /tmp
mkdir A
mkdir B
ln -s /tmp/A /tmp/B
Creates a link: /tmp/B/A that points to /tmp/A
However, if /tmp/B/A exists you get an error stating:
Code:
ln: creating sybolic link '/tmp/B/A' to '/tmp/A': File exists