attempt to call nil

I've just made a small turtle sap farm program but when i trie to run i get: "startup:24:attempt to call nil".
Does someone know how to fix this?
Script: http://pastebin.com/Cbx9Sv9d

Split into new topic.

It's sleep, not turtle.sleep.

use sleep(900) instead of turtle.sleep(900).

thank you very much