function reFuel()
    if turtle.getFuelLevel() < 20 then
         for i=1,5 do -- you were missing a 'do' here.
            turtle.refuel()
         end
    end
end

function uTurn()
    turtle.turnLeft() -- it's turtle.turnLeft() not ternLeft ;p
    turtle.turnLeft()
end

function toTree()
    for i=1,3 do
       turtle.forward()
    end
end

function cutTree()
    turtle.dig()
    turtle.forward()
    while turtle.detectUp() do
        turtle.digUp()
        turtle.up()
    end
    while not turtle.detectDown() do
        turtle.down()
    end
end

function toHome()
    uTurn()
    for i=1,3 do
       turtle.forward()
    end
    turtle.select(2)
    turtle.select(3)
    turtle.drop()
    turtle.select(1)
    end

function replantTree() -- you were missing the'()' here.
    turtle.select(1)
    turtle.place()
end


--main script--
while true do
reFuel()
toTree()
cutTree()
replantTree()
toHome()
sleep (300)
end