Cannot Join A Fellow's Server

Hello, every time I try to connect with my fellow, it'll be logging in..., Loading Terrain, and then Disconnects Me. We are using 1.12.2 and I am using the correct version, however I cannot join the server. Strange thing is, I can join my own server but not my fellow's. How do I fix this? I've went through my firewall and allowed all the Java SE Library apps.

All it shows is

Connection Lost

Disconnected

How am I supposed to give you the server log from my friends server?

Okay now it works. Don't know why it wasn't allowing me to join last night, but now it is.