Kotlin: error: unresolved reference 'x' (Unresolved reference: x)

unresolved reference 'x' means the Kotlin compiler found no declaration named x that is visible from that line: no local variable, member, import or dependency provides it. The usual causes are a typo, a missing import, or a library that is not on the module's classpath (a dependency missing from build.gradle.kts, or a Gradle sync that has not run). Fix the name, add the import, or add the dependency and rebuild.

What it looks like

Output of the Kotlin 2.3 command-line compiler (K2) for a file with a typo and an import from a library that is not a dependency:

import kotlinx.coroutines.launch

fun main() {
    val name = "Ada"
    println(nmae)
}

Main.kt:1:8: error: unresolved reference 'kotlinx'.
import kotlinx.coroutines.launch
       ^^^^^^^
Main.kt:5:13: error: unresolved reference 'nmae'.
    println(nmae)
            ^^^^

The K1 front end (Kotlin 1.x, or Kotlin 2.x with -language-version 1.9) prints the older wording: error: unresolved reference: kotlinx and error: unresolved reference: nmae. IntelliJ IDEA and Android Studio report the same diagnostic in the editor.

Fixes

A dependency for a JVM or Android module:

dependencies {
    implementation("org.jetbrains.kotlinx:kotlinx-coroutines-core:1.10.2")
}

In a multiplatform project, add it to the source set that uses it (commonMain.dependencies { ... }); code in commonMain cannot see a dependency declared only for jvmMain.