For the first year WhatsApp is free and after a year it expires so if you are getting any notification regarding WhatsApp expiry you don’t need to worry...!
After the expiry date, what’s app asks you to purchase WhatsApp application for $0.99 that’s really not a problem as WhatsApp is totally worth it to be purchased for $0.99.
You can check your WhatsApp expiry by going to your WhatsApp setting >Account>payment info you will find a service expiration at the top of your screen and you will see the date on when your WhatsApp is going to expire.
If you wanna expand your expiry date without spending any amount I will suggest you read this article : Extend whats app expiry date