What am I missing?

build.gradle.kts

plugins {
    java
    kotlin("multiplatform") version "1.3.72"
    kotlin("plugin.serialization") version "1.3.72"
}

group = "org.example"
version = "1.0-SNAPSHOT"

repositories {
    mavenCentral()
}

dependencies {
    implementation(kotlin("stdlib"))
    implementation("org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.20.0")
    testCompile("junit", "junit", "4.12")
}

kotlin {
    /* Targets configuration omitted. 
    *  To find out how to configure the targets, please follow the link:
    *  https://kotlinlang.org/docs/reference/building-mpp-with-gradle.html#setting-up-targets */

    sourceSets {
        val commonMain by getting {
            dependencies {
                implementation(kotlin("stdlib-common"))
                implementation("org.jetbrains.kotlinx:kotlinx-serialization-runtime:0.20.0")
            }
        }
        val commonTest by getting {
            dependencies {
                implementation(kotlin("test-common"))
                implementation(kotlin("test-annotations-common"))
            }
        }
    }
}