I think there is a limit to the number of primary partitions you can have and trying to add another OS will bring it up to 5. Can you delete a couple of later partitions and let the new OS do the work?
Samsung Galaxy S8 review
1995-2015: How technology has changed the world in 20 years
This poetic animation about monsters is illustrated in a Van Gogh painterly style
Apple predictions 2017
Synology invites you to visit them at Insomnia on 26th – 29th August. Book your tickets now and visit Synology.