Unfortunately, most solutions or fixes for this error ask you to deal with the network adapter in your computer, but since you have an Internet connection that's active, those solutions are no longer viable.
My recommendation is to install ES File Explorer to archive the WhatsApp folder on your storage, then upgrade Bluestacks to the latest version. This is known to fix a lot of known errors with the application.
Secondly, install a manual update of WhatsApp application.
