Double-check your syntax - if you are making a mistake with that you will
sometimes get that error message...

Checked the syntax, its fine. I feel its something to do with IIS.

Here is the syntax what I enter :

stsadm.exe -o addwppack -filename xyz.cab -globalinstall -force

Nope, it's your syntax - you are missing the -url flag which is needed for
addwppack but not for enumwppacks.

-url flag required only when the web  part is being deployed  to a specific
virtual server.

Without -url flag will deploy  the web part  on all the available virtual servers
on the IIS.

In my case, we have 2 virtual servers, so tried without -url flag.

But anyways I tried with -url flag, still no luck ...

you are right -url is an optional parameter in addwppack. We don't have
to specify the url.

Does this SPS machine have multiple IIS virtual servers on it? Are you
using Host Headers or IP Address to differentiate the virtual servers?

Finally was able to solve  the problem...

Solution : Run stsadm.exe -upgrade