Can’t send WhatsApp messages on iPhone SE 3rd generation

WhatsApp is not an Apple product. Contact the app developer here --> App Support or WhatsApp FAQ  Or Restoring your chat history - WhatsApp FAQ or Contact WhatsApp

WhatsApp Business is not an Apple product. Contact the app developer here --> Developer Website  or App Support  or WhatsApp business products - WhatsApp FAQ

Sometimes the server has issues and it affects worldwide so it might not always be a problem on Apple side.

And, if the problem persists, you can always delete the App, and install it again.