[ovirt-users] Failure to upgrade Cluster Compatibility Version

I have been upgrading my oVirt platform from 3.4 and I am trying to get to 4.2. I have managed to get the platform to 3.6, but need to upgrade the Cluster Compatibility Version. When I select 3.6 in the Cluster Compatibility Version and select OK, it highlights Compatibility Version in red, (image attached). There are no errors been displayed on screen, or in the /var/log/ovirt-engine/engine.log file. Please let me know if I am missing something and how I can resolve this? Thanks Jonathan