You make soap from oils and NaOH, but not all oils will work. They have to be triglycerides. Triglycerides have a glycerol head that 'ties' the three fatty acid tails. It is a distinctive type of oil. Google image triglycerides and you will see the shape.
Tea tree essential oil is an oil but it is not a triglyceride, hence, it cannot be reacted with lye to make soap. You can add tea tree essential oil (up to 3% or so) to a soap made with soaping oils (triglycerides) but not make soap out of it.