Whilst setting up my Exchange 2016 lab environment I came across this error:

Go to Active Directory Users and Computers. Turn on View > Advanced Features. Find your DAG computer name in your AD structure and Right Click > Properties

You can see the DNS name is missing from my DAG computer account. Go to the Attribute Editor tab.

Edit the Attribute “dNSHostName” to reflect your domain etc:

Now reattempt adding the computer to the DAG.

YEY it worked:

Something else you may want to check is this article from technet: