Type mismatch inferred type is Unit but Void was expected

Kotlin uses Unit for methods which return nothing instead of Void. It should work

fun testA(str: String, listner: (lstr: String) -> Unit) {

}

According to Kotlin documentation Unit type corresponds to the void type in Java. So the correct function without returning value in Kotlin is

fun hello(name: String): Unit {
    println("Hello $name")
}

Or use nothing

fun hello(name: String) {
    println("Hello $name")
}

If you do need Void (it's rarely useful, but could be when interoperating with Java code), you need to return null because Void is defined to have no instances (in contrast to Scala/Kotlin Unit, which has exactly one):

fun testA(str: String, listner: java.util.function.Function<String, Void?>) {
...
}

testA(("hello") { lstr ->
    print(lstr)
    null
}