at the beginning of the chapter why is lennie in the barn?

Possibly, he found a nice, soft creature to strokie... or he got into a perdicament, that caused him to hide. That is what I've been told by my fellow readers. :)

He is there to play with his pup. He would rather pet a soft thing than play horseshoes with the other men. This effectively isolates Lennie so that the scene with Curley's wife is a private one.