Why?

[Frostbit]

What is a friend level? I can’t play with my friend because my friend level is too high? What does this mean?

[Castiel]

This is referring to your wolf's level, which can be seen on the top right next to your wolf icon. All lobbies have a minimum and maximum level limit (e.g. 30-50) to avoid rooms being too strong or weak for certain players.

You can view the level of a friend's room when you click on their user on your friends list. It should appear on the right.

In this case your wolf wasn't strong enough for your friend's lobby.

[Unknown Content]

Hey just switch to an Asia server and tap Green hills then you be sent in 10-50 server. As long you'll level 10 or higher and the server is empty your friend can join