If you use Whatsapp to send messages to your friends and acquaintances, it may be worth knowing that the system requirements are increased.
As of January 1, it is IOS 12 or later that applies, which means that older models such as iPhone 5 and 5C are no longer supported.
If you have a newer model than that, you may need to update the system, which you do by opening the app Settings and head to General -> Software Update. Then just follow the instructions until the installation is complete.
If you prefer Android, version 4.1 or later applies.