Why am I unable to create this Availability Group? (error 35220)
Here's the blog where I found the answer: https://bahtisametcoban.home.blog/2019/01/04/always-on-availability-groups-with-multi-subnet-failover-cluster/.

In case you don't feel like reading through it, the answer is


  If you enable [Availability Groups] before cluster is fully operational and ready  you will get an error when you try to create AG


When I installed and configured SQL Server on the machines in question, I jumped the gun on enabling the Availability Groups feature (SQL Server Configuration Manager-&gt;Services-&gt;SQL Server-&gt;Properties-&gt;AlwaysOn High Availability tab) and checked the box (and restarted the service) before the WSFC was created.  This is what caused the error when I tried to actually create an AG.

The fix is easy: just disable Availability Groups, restart the service, then re-enable them and restart it again.

Hope this has been helpful!