I've been struggling with this error like forever. It must be some weird IntelliJ bug, because, at the beginning, when I went to File > Project structure > Platform settings > SDKs, it picked up the Oracle OpenJDK 21 that I had installed on my computer, but the Classpath tab was empty, it just said: nothing to show. And I had this error Kotlin: Cannot access 'java.io.Serializable' which is a supertype of 'kotlin.String'. Check your module classpath for missing or conflicting dependencies showing all the time. What I did was, from File > Project structure > Platform settings > SDKs, remove the JDK, then add it again from the directory where I had it installed, and then the Classpath tab showed some entries and the error went away.