powershell command enable-alwayson not work for SQL Server Named instance
Check out this documentation and tweak your command slightly.

https://docs.microsoft.com/en-us/sql/database-engine/availability-groups/windows/enable-and-disable-always-on-availability-groups-sql-server?view=sql-server-2017

The example shows this as the proper command for an instance

Enable-SqlAlwaysOn -Path SQLSERVER:\SQL\Computer\Instance