As part of the migration to Exchange 2016 you might have to recover an Exchange 2010 member server or both if you have storage corruption or the VM’s OS has simply become unusable.
Recovery is done and when you try add the member server/s back to the Database Availability Group (DAG), you presented with a simple error.
The DAG computer object in Active Directory has not been disabled and so it fails.
Really simple fix, go to ADUC (Active Directory Users and Computers) and right click on the computer object and select disable.
Now click the back button on the member screen and it should install Failover Clustering successfully and add the members to the DAG.
Hope it helps.