You wouldn't happen to have an LG V30 phone would you? Don't ask me why I'm asking you. Let's just say I burned a few hours over the course of two days getting my phone to work. My wife on the other hand - her Google Pixel 2 worked right off the bat. Same cable. And that was a good test... the cable and the head unit works, but my phone was not happy.
I'll tell you how I have my phone set up, and maybe you can configure yours similarly and re-try.
1). After installing AA, and without it connected to the USB cable, launch it.
You'd better see some blocky rounded rectangles popup. In other-words the program shouldn't launch and then disappear on you.
2). Choose the 3-horizontal lines upper left (the Menu Icon).
3). Select Settings
4). For General,
I have Limit Wi-Fi off.
When I select Accepted Cars, I have "Subaru" selected. For Rejected cars, I have None selected.
5), For "OK Google" detection, currently I only have "while driving" turned on
For Google Assistant, I haven't made any changes yet.
6) For Autolaunch, I have it selected OFF. The rest of the settings won't matter if it's OFF.
7) For System Google Analytics, I have that turned on. I think Google needs a lot of data to really hone this app, specially when it crashes. YMMV
Now then, for me? I think the thing that helped my phone to work were two-fold.
A), I power on the head unit. I wake up the phone. I do not have to explicitly launch AA on my phone. It does so when I connect the USB cable.
B). I have gone into Settings on my Phone. General | Apps & notifications | Special Access|Unrestricted Data Access, then when the App list comes up, I enable Android Auto to have unrestricted Data Access.
Reboot your phone.
Now I seem to be able to get my phone to connect to the head-unit if I do or if I don't launch AA. You can try it both ways.
If I see a message on the HU to plug and unplug my phone, I just ignore it. A few seconds later, AA will launch on the phone, and the Head Unit display will be taken over. On my LG, and I don't know why but I think AA is not fully baked yet, even though I've told it before it has all priviledges, it asks me to Allow or Deny access. I can just refuse to answer that question on my phone, and things still work fine. I can do an Accept, and that works fine too. I do not think you should select Deny.
If I go into AA on the phone, and select it's settings, I am running Ver 4.4.592334-release. You might try uninstalling and reinstalling the app again from the Google PlayStore, set the settings like mine and perform one more reboot then plug your cable in..
Let me know if this has helped you or not. Trust me, others are having problems with AA, and they had working versions until recently, but I think they did a new release a few days ago. Hopefully they undid what they did.
Good luck.