Kotlin not configured in IntelliJ
Trouble Shooting history
When I create spring-boot with Kotlin, IntelliJ can’t reconize Kotlin class.
As a result, I added the standard library to dependency management.
implementation("org.jetbrains.kotlin:kotlin-stdlib")
Kotlin-stdlib has extended versions of the standard library that add support for some of the features of JDK 7 and JDK 8.
Checking Kotlin language official page this link and choose apropriate one.