please help

Old versions of network simulators will require patching before the installation.

So before run the ./install, you have to apply suitable patch for that version (ns2.1b8a).

Try to find a patch file called ns2.1cygwin.patch.
you may make ns2.1b8a to work by applying this patch and with some little further modifications.

Once I made it (ns-allinone-2.1b9a) work on cygwin.