After doing a search for notification you could have found several extensions, which I listed a while ago also here How do I install extensions? - #13 by Taifun - Extensions - MIT App Inventor Community
And nowadays let me suggest you to use 🔔 MelonNotification • Various styled notifications + Itoo - Extensions - MIT App Inventor Community for immediate notifications
For notifications at a specific datetime in the future, use App Inventor Extensions: Alarm Manager | Pura Vida Apps
Taifun