WhatsApp can't be installed
Reading all the questions associated with this type error, you get the impression it will require root to enable you to delete the app from the /system partition if it remains.

Previous to doing that, I would suggest trying this, just to be sure it is not a file or folder remnant getting in the way:


  Some apps leave remainders during uninstalls that aid in the
  restoration of state or level information upon re-installing a later
  version. Usually these remains are unimportant to a user who does not
  intend to re-install or to any other user where consistent state or
  level data is unimportant. These remains have a particular name in an
  app that can clean them up: "corpse files".
  
  SDMaid
  is a disk cleaner that can locate, and delete, files related to
  corpses. Install the app, and select CorpseFinder from the menu to
  locate, and, optionally delete them.


DISCLAIMER: I am not the developer, just a satisfied user.

If this is not the solution, see the answers for more intensive actions through other questions here.