A few things:
1.) I'm assuming you're working with 9.6 patched with SP1, and all available post SP1 patches for your core.
2.) A join domain action should not really be the first action that you perform in the system configuration section. It should be to install the agent.
3.) Also you should configure a "provisioning agent" with bare minimum features, and all the scheduled items turned off. This way the agent doesn't walk on itself during your later actions of sys config. Using a standard agent config could lead to action failures just because the agent decides to do a bunch of other things while you're asking it do the actions that you really want.
4.) If the agent installation is failing it is usually related to how the server name is setup in the WIM file for the win PE environment. It is usually short name. You may need long name instead, or vice versa. You can test this without changing the WIM file though. Try adding both the short name for the core, and the FQDN for the core EACH as "preferred servers". These are essentially fake entries. You can even provide narrow IP ranges on these two fake preferred server entries. like 1.1.1.1 -1.1.1.1 and 1.1.1.2 - 1.1.1.2. You don't need to do any replication stuff with these at all, because they are basically the core, so you would never replicate from itself to itself anyway. The important thing is that you provide both of these preferred server entries with the username and password for READ access that is needed to be able to connect to the core's UNC shares to get the agent installation. What is probably happening now is that is failing because it isn't finding your server. This might be related to your join domain step not working, but again.. agent installation should not be dictated by the success of a join domain step anyway. You can figure this out if you go into your provisioning template and uncheck the box that makes it remove the provisioning folder. This way you can review the logs in this folder to see the errors. The ldprovisioning folder will be in the root of the C Drive. After you run this task again, and it dies again (assuming it does!), then you can check the log files in that folder.
If you read closely, in the agent installation action window, it states that it uses preferred server credentials, so if you enter the server as long and short name in preferred servers, then the provisioning action will be able to succeed.
Good luck!