'Unresolved reference' errors for android library module referenced in app module

Found the problem, my android library module was missing the kotlin configuration:

apply plugin: 'kotlin-android'

dependencies {
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jre7:$kotlinVersion:<version>"
}

Although I used kotlin .kt files in it, it could build without and also

Tools -> Kotlin -> 'Configure Kotlin in projects'

had told me 'All modules with Kotlin files are configured'

Your module's build.gradle file should have:

apply plugin: 'com.android.library'
apply plugin: 'kotlin-android'
apply plugin: 'kotlin-android-extensions'

dependencies {
    ...
    implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"
    ...
}

In my case It was apply plugin: 'kotlin-android',

implementation "org.jetbrains.kotlin:kotlin-stdlib-jdk7:$kotlin_version"

and also added it on build.gradle

androidExtensions {
  experimental = true
}