However, the Spark2 service never starts and when I attempt do start it manually through the Cloudera Manager interface, I get the following error : "This role requires the following additional parcels to be activated before it can start: [spark2]."
Do you have any idea of what is missing in the configuration ?
That conf looks right to me. Though the service should be SPARK2_ON_YARN instead of SPARK2-ON_YARN. Also can you verify that you hava Java 8 installed which is a requirement for Spark 2, see here for additional details.