This always seems to give me trouble, and I always forget the manner in which I run this that works correctly, so, viola!

Invoke-Command -ComputerName $name -ScriptBlock{
        Import-Module SqlPS -DisableNameChecking

        $srv = New-Object Microsoft.SqlServer.Management.smo.Server "."
        Enable-SqlAlwaysOn -InputObject $srv -Force
        $srv.ConnectionContext.Disconnect()
    }