I was attempting to create an Availability Group on a SQL Server 2017 instance and was running into error 35220:
Could not process the operation. Always On Availability Groups replica manager is waiting for the host computer to start a Windows Server Failover Clustering (WSFC) cluster and join it. Either the local computer is not a cluster node, or the local cluster node is not online. If the computer is a cluster node, wait for it to join the cluster. If the computer is not a cluster node, add the computer to a WSFC cluster. Then, retry the operation.
I was stumped, because the host was most definitely part of a WSFC, and the node was online. I eventually found the answer in a blog that was focused on Multi-Subnet Availability Groups, which I hadn't looked at earlier because mine was not Multi-Subnet.
Thus, I'm posting this here to give better visibility to the solution in case some other poor DBA gets stumped the same way I did. I will post the solution below.