Get Selected Node Index number

Thank you for replaying.

You are absolutely right.
And was doing this wrong, must understand how this works later.
I was trying yo create a macro that changes the first Node.

My code is:

Sub ChangeFirstNode()
    Dim s As Shape
    Dim sp As SubPath

    Set s = ActiveShape
    Set sp = s.Curve.SubPaths(1)

    Dim n As Long
    
'   I wanted to pass the Node Index value to a variable
'   but I get a error "Object doesn't support this property or method"

    n = s.Selection.FirstNode.Index
    
    If sp.Closed Then
        sp.Nodes(n).BreakApart
        sp.StartNode.JoinWith sp.EndNode
    End If
End Sub

Changing my code, removing this part:

    Dim n As Long
    
'   I wanted to pass the Node Index value to a variable
'   but I get a error "Object doesn't support this property or method"

    n = s.Selection.FirstNode.Index   

And changing this one to:

   If sp.Closed Then
        sp.Nodes(ActiveShape.Curve.Selection.FirstNode.Index).BreakApart
        sp.StartNode.JoinWith sp.EndNode
    End If

It works. So, I'm missing something in the code. I'm learning as I go.

Depending on what you want to do with it, you might not need the index of the node. After identifying a node, you can use it to refer to the subpath that contains it:

Node.SubPath

Are you trying to change where a closed subpath starts/stops, by selecting the node you want, and then running a macro? You could look at this for a start.

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Sub change_subpath_start_node()
Dim s As Shape
Dim spThis As SubPath
Dim nodeThis As Node

    Set s = ActiveShape
    Set nodeThis = s.Curve.Selection.FirstNode
    Set spThis = nodeThis.SubPath
    If spThis.Closed Then
        nodeThis.BreakApart
        spThis.Closed = True
    End If
    
End Sub

That doesn't have anything in it to make it "safe", of course!

'n = s.Selection.FirstNode.Index'   does not have any meaning for your code...

It should have a meaning changing it in 'n = s.Curve.Selection.FirstNode.Index. Exactly as Eskimo suggested.

Even in this way, to have a meaning and not returning an error you have to select (at least) a node...

Using Shape Tool (F10) and selecting it (them).

Without this, maybe it is good to explain what you exactly want. Even if you try to play and learn...

What do you select and what do you intend to do with that selected shape...

To be more spectacular (of course, on my taste...) the above Eskimo's code can be transformed in:

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Sub change_subpath_start_node_bis()
Dim s As Shape
Dim spThis As SubPath
Dim nodeThis As Node

    Set s = ActiveShape
    Set nodeThis = s.Curve.Selection.FirstNode
    Set spThis = nodeThis.SubPath
    If spThis.Closed Then
        nodeThis.BreakApart
        nodeThis.Move s.Curve.Nodes.last.PositionX - nodeThis.PositionX, s.Curve.Nodes.last.PositionY - nodeThis.PositionY
    End If
End Sub

Of course you must select a node which will be moved to the last node of the shape...