If you are trying to have a

() -> CreateEmail

, you need to provide an else branch:

val createEmail = {
    if (type == Type.ERROR) {
        dataProvider.createErrorEmail(id)
    } else {
        // some other CreateEmail
    }
}

or use a when statement similar to what @Nolan suggested, but wrapped in braces:

val createEmail = {
    when (type) {
        Type.ERROR -> dataProvider.createErrorEmail(id)
        // Other branches
    }
}

You can also specify the type explicitly to get a compiler error and avoid type inference altogether:

val createEmail: () -> CreateEmail = ...