That “Unofficial Whatsapp Account …. Download Official Whatsapp” error pops up when you’re using a modded version 
WhatsApp blocks them now , (i had same issues…).
Just:
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Uninstall the modded app completely
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Download the official WhatsApp from Play Store
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Install and try logging in again
Might need to wait a few hours or 1 day if it got flagged.
!