On some phones older WhatsApp cannot be installed because of an already existing hidden WhatsApp, not visible among list of apps due to vendor choices or other reasons (ie. Android 10 "Keep data" or Samsung Secure Folder).

You can check this through WazzapMigrator's feature Check Whatsapp installed

How to fix this:

  1. reinstall WhatsApp (older or from Play Store, depending on the one you had)
  2. uninstall it immediately after (see interactive tutorial below)