“The computer account ‘DAG1’ could not be validated. Access was denied. Check that the current user (NT AuthoritySystem) has permissions to create computer accounts in the domain or to claim the computer account”

Scenario:  When attempting to add your first server to a new DAG, you may receive the following error:

A server-side database availability group administrative operation failed. Error The operation failed. CreateCluster errors may result from incorrectly configured static
addresses. Error: The computer account ‘DAG1’ could not be validated. Access was denied. Check that the current user (NT AUTHORITYSYSTEM) has permissions to create
computer accounts in the domain or to claim the computer account.

Solution:  Once the DAG  Computer Account (CNO) is created and disabled, perform the following:

1. Assign full control permission on the DAG CNO of the first Exchange Server you are trying to add to the DAG.
2. Assign full control on the DAG CNO of Exchange Trusted Subsystem.

Try it again.