Error when demoting an Active Directory domain controller: This domain controller holds the last replica of the following application directory partitions: DC=MSTAPI,DC=yourdomain,DC=com
Active Directory has the following partitions: Application partition, Configuration partition, Domain partition, and Schema partition. The Application partition is used to store data from Active Directory-integrated software. This error indicates that an Application partition exists on this DC. There are two possibilities: this is the last DC in the domain or it is not.
If this is the last DC in the domain, and the domain information is no longer needed, then it is safe to delete the replica.
If this is not the last DC and you require the Application partition, you must remove the DC from the Application partition’s replica set. Use ADSIEdit and consult Microsoft’s help to perform this operation.