Can’t use keyboard in WhatsApp
All components of the keyboard are controlled entirely by the iOS, including multi-language keyboards, Auto-Correction, Check Spelling, and Predictive text. These features can be turned on and off in iPhone Settings > General > Keyboard. If these features aren't working, this is an issue with the iOS, not WhatsApp.
If you added another keyboard, you can access this by tapping on the globe icon in the bottom corner of the keyboard.
If you’re experiencing slowness in typing, changing keyboards, or adding emoji, please reset your keyboard dictionary in iPhone Settings > General > Reset > Reset Keyboard Dictionary.
To learn more about the keyboard on your iPhone, please visit the Apple Support website.