What does this error mean?

This error means Cassandra was not able to retrieve the process ID (pid) for the Cassandra JVM from the operating system.

Why does this error occur?

This error occurs when the Java NativeLibrary.getProcessID() and the ManagementFactory.getRuntimeMXBean() both failed to retrieve the Cassandra pid from the underlying operating system. 

How do you fix this error?

Since the pid exists and Cassandra is running, there is nothing to fix from a Cassandra perspective. Check the underlying OS for CPU or memory issues that could have prevented the OS from returning the pid without timing out.