Class type parameters cannot be reified, so they cannot be passed to generic methods with reified type parameters.

You should store a Class<X> in CSVMigrator, or a TypeReference if X itself can be a generic type. Then write a non-reified version of getIterator and pass those to a different overload of readerFor that takes Class<T>/TypeReference.

class CSVMigrator<X: Serializable>(
    val clazz: Class<X>,
//  or:
//  val typeReference: TypeReference<X>,
)

private fun <T> getIterator(
    reader: Reader,
    clazz: Class<T>,
//  or:
//  typeReference: TypeReference<T>,
): MappingIterator<T>? {
    val schema = CsvSchema
        .emptySchema()
        .withLineSeparator("\n")
        .withColumnSeparator(',')
        .withQuoteChar('"')
        .withHeader()

    return csvMapper
        .readerFor(clazz /* or typeReference */)
        .without(StreamReadFeature.AUTO_CLOSE_SOURCE)
        .with(schema)
        .readValues<T>(reader)
}

Usage:

val iterator = getIterator(this.clazz /* or this.typeReference */)

When creating a CSVMigrator, pass in the Class or TypeReference.

CSVMigrator(UserDto::class.java)
// or: (not sure if Jackson for Kotlin has an easier to create a TypeReference)
CSVMigrator(object: TypeReference<SomeGenericType<String>>() {})

The simplest way to solve this is not to use reflection at all. You can pass the property values directly to the class constructor:

data class Member(val firstName: String, val lastName: String)

fun readCsvFileKotlin() {
    // ...
    for (line in reader) {
        val mbrProperties = line.split(",") 
        memberList.add(Member(mbrProperties[0], mbrProperties[1]))
    }
}

If you want to set properties through reflection, you need to declare them as var rather than val, so that they can be changed after the object is created. Then you can use the set method on the property object to change the property value:

data class Member(var firstName: String? = null, var lastName: String? = null)

fun readCsvFileKotlin() {
    // ...
    val prop = Member::firstName
    for (line in reader) { 
        val mbrProperties = line.split(",") 
        val member = Member()
        prop.set(member, mbrProperties[0])
        memberList.add(member)
    }
}

This can be achieved using basic Kotlin instead of an external library.

data class

data class Category(
    val id: String,
    val name: String,
)

utility function

fun <T> csvOf(
    headers: List<String>,
    data: List<T>,
    itemBuilder: (T) -> List<String>
) = buildString {
    append(headers.joinToString(",") { "\"$it\"" })
    append("\n")
    data.forEach { item ->
        append(itemBuilder(item).joinToString(",") { "\"$it\"" })
        append("\n")
    }
}

usage

fun main() {
    val firstCategory = Category(0, "Phone")
    val secondCategory = Category(1, "Laptop")
    val categories = listOf(firstCategory, secondCategory)
    val csv = csvOf(
        listOf("id", "name"),
        categories
    ) {
        listOf(it.id.toString(), it.name)
    }
    println(csv)
}