Error: Exception in thread "main" java.lang.NumberFormatException: For input string: "kotlin.Unit"

Remove the .toString().toDouble() part and then implement it this way to fix the decimal formatting problem:

fun calculateWeight(bodyWeight: Double) {
    val mercury = bodyWeight * 0.38
    val venus = bodyWeight * 0.91
    val earth = bodyWeight * 1.00
    val mars = bodyWeight * 0.38
    val jupiter = bodyWeight * 2.34
    val saturn = bodyWeight * 1.06
    val uranus = bodyWeight * 0.92
    val neptune = bodyWeight * 1.19
    val pluto = bodyWeight * 0.06

    println("My body weight is %.2f pounds and on the different planets it equals:\n" +
            "Mercury: %.2f, \nVenus: %.2f, \nEarth: %.2f, " +
            "\nMars: %.2f, \nJupiter: %.2f, \nSaturn: %.2f, \nUranus: %.2f," +
            "\nNeptune: %.2f, \nPluto: %.2f".format(
                    bodyWeight, mercury, venus, earth, mars,
                    jupiter, saturn, uranus, neptune, pluto))
}

Better yet, here's a more idiomatic way to do it.

enum class Planet(val relativeGravity: Double) {
    Mercury(0.38),
    Venus(0.91),
    Earth(1.00),
    Mars(0.38),
    Jupiter(2.34),
    Saturn(1.06),
    Uranus(0.92),
    Neptune(1.19),
    Pluto(0.06);

    fun convert(weight: Double): Double {
        return weight * relativeGravity
    }
}

fun calculateWeight(bodyWeight: Double, unit: String = "pounds") {
    val prefix = "My body weight is $bodyWeight $unit and on the different planets it equals:\n"
    println(Planet.values().joinToString(prefix = prefix, separator = ",\n") { planet ->
        "%s: %.2f".format(planet.name, planet.convert(bodyWeight))
    })
}