Why can't I join a friend's world on Minecraft PE?

You must be connected to the same network to play together, unless you use a third-party app.

There's a third-party app called Multiplayer for Minecraft PE that allows you to host a server and connect to other networks. It's definitely available for iOS, but I'm not sure about Android or other app stores.

The other option is to connect to the same wi-fi network that the other person is on. This will require you to be at the same physical location as him.

I play Minecraft on PC but I'm pretty sure you would need a server adresss before you try it.

Open Minecraft PE, check your internet connection. If it's down restart your router/modem.

As they said you can play on the same network or same server ,but I play with my friend normally with xbox live accounts we add each other as friends on minecraft and whenever someone plays it's showing which world he's or she's playing but sometimes it doesn't work due to connection and some other unknown problems but if it didn't work we just play on the same server

Hope my answer helped:)

uninstalling and reinstalling the game usually helps me if your using xbox live if the world doesnt show up inviting the other person to it usually works it might require a game close or two or maybe it works first try servers are another option but thats all i know idk man xbox live is weird