Several iPhone models, when downloading the new version of WhatsApp , do not meet the requirements requested by the app to carry out said update.
From WhatsApp they announced that its latest version (2.21.50), which is already available in the App Store, requires the operating system iOS 10 or later for its installation. IPhone users with previous operating systems are prevented from downloading this latest version, which incorporates personalized wallpapers, a sticker search engine, as well as improvements in temporary messages. But not all is lost.
First you have to check which version of the system is installed. To do this you have to go to "Settings" and then enter "General". Pressing "Information" you can see the version of iOS loaded. If you have a version older than 10, you can update it manually.
With iPhone 5 or later, you can update to iOS 10.3.4 from Settings> General> Software Update. With this update, the user ensures that he can download the new version of WhatsApp without problems. With iPhone 7 or higher, they can update to iOS 14.4.1 which is Apple's most up-to-date operating system to date.