This issue occurs when you download an app from a region that originally supports the app, but later stops supporting it. As a result, you are unable to update the app even if you change your App Store region.
For instance, if you downloaded the app from the Chinese App Store and later switched to the US App Store, you may still be unable to update it and receive an error message.
The solution is as follows:
- Switch your App Store region to a country that supports the app (e.g., US, UK).
- Go to Settings > General > iPhone Storage.
- Find and select WhatsApp.
- Tap Offload App (this will delete the app but keep its documents and data on your phone).
- After offloading the app, you will see an option appears as Re-install App. Tap it, and WhatsApp will be reinstalled in a few seconds with all your data remains as origianl.
Hope it helps.