You have an Active Directory Domain Services (AD DS) domain that contain s the domain...

In the Windows Server hybrid administrator objectives for AD DS, application directory partitions (also called “naming contexts”) are forest-wide objects whose creation and deletion are controlled by the Domain Naming Master FSMO. The AZ-800 materials explain that operations which “add or remove naming contexts in the forest” must be performed on, or reachable by, the DC that holds the Domain Naming Master ; if that role holder is unavailable, the operation fails until connectivity is restored or the role is transferred/seized. Because your custom application partition belongs to the forest schema of naming contexts, removing it requires contacting th e Domain Naming Master. From the table, DC3 holds the Domain naming master role. Therefore, a failed attempt to delete the partition indicates the Domain Naming Master is not available, which points to DC3 being unavailable.