If you missed the memo over the summer, as of 14 August 2018, hybrid mobile device management (Intune integrated with Configuration Manager) is a deprecated feature with support ending on 1 September 2019. For more information on what this means, check out this documentation.
The short version: Intune standalone is now Microsoft's recommended deployment topology and it is required for Co-Management.
From this point forward, if you want to enable Co-Management, you must move from an Intune hybrid configuration to Intune standalone.
If, at any point in the process, you need assistance migrating away from Intune hybrid, just use the "request assistance" option at: Microsoft.com/en-us/FastTrack/