solaris 10- cant create softlink

bash-3.00# ln -s /lib /usr/lib
ln: cannot create /usr/lib/lib: File exists
bash-3.00#

Which means there is a lib directory inside the /usr/lib as lib .

No, it means that the destination directory /usr/lib exists.

If /lib does not exist as any type of file and you want to create the soft link.

lr-xr-xr-t   1 root       sys              8 May  7  2003 /lib -> /usr/lib

On most unix systems this soft link exists already.

The command is:

ln -s /usr/lib /lib

That's not true for Solaris 10. That's why I pointed out that error

There is no error but yours.
There are already both /lib and /usr/lib directories under Solaris with different contents.
What are you trying to achieve ?