You have selected the Power DNS option within Nameserver Selection but you are seeing the error message: The "pdns" service is not configured.
You select Power DNS but it still shows as not configured when viewing the Nameserver Selection interface in WHM.
To correct his you will want to run the following command via the command line as the root user. This will create the touch file that indicates Power DNS should be in use.
# touch /var/cpanel/usepowerdns