I also got that error until I got rid of the "else:" line. If you write "else," you only return cost if you rent the car for less than three days, but we always want to return the cost.
The cost is calculated right away at the top (cost = 40 * days). Then a discount is applied if relevant. If no discount is relevant, there is no "else" thing you need to do. We want that return statement to apply always, not just if there were no discount. Hope that makes sense.
In other news, It looks like you also need to change your - to -=. This will be equivalent to the += shown in the example. (If not you will probably get the error "It looks like rental_car_cost returns 120 instead of the correct amount (100) for 3 days.") We weren't directly taught what += and -+ mean, but it looks like it is shorthand for adding or subtracting to itself. So I think cost -= 20 means the same as cost = cost - 20.