I want to mention that I had completely filled and submitted "Visa
credit card joining form" for purchase of 'paid membership' at the
following website:

http://netverifier.com/join/join.asp?siteid=

but after clicking its 'submit' button that credit card joining form
"returned" and it could not be processed, I want to know that "why" it
could not be processed?

NOTE: Please note that my Visa credit card is "VALID and active and I
had filled that Visa credit card joining form CORRECTLY".

It might be worth trying again. Sometimes a site like this may have
problems behind the scenes.

I've certainly experienced this at major internet retailers in the
past, including Network Solutions who really should have better
infrastructure. One day nothing will process, the next it all works
fine.