Where am I going wrong with this code?

Solution 1:

Looking at the Turtle API you will see the getFuelLimit() is only implemented in version 1.6, and you are running version 1.57.

From the wiki:

Added by version 1.6 of ComputerCraft, this command returns the maximum amount of fuel a turtle may store. By default, a regular turtle may hold 20,000 units, and an advanced model 100,000 units; both values can be changed in ComputerCraft.cfg.

In builds prior to 1.6, turtles may store practically unlimited amounts of fuel.

Solution 2:

to make it forward compatible you can do the following:

function getFuelLimit()
    if turtle.getFuelLimit ~= nil then
        return turtle.getFuelLimit()
    else
        return 0;
    end
end

and add a check for 0 when checking the fuel against it or set your own fuel limit