Change paddings using VBA

[Platypuce]

Hello,
I’m new to VBA, and I’m working on Powerpoint 2016. I’m looking to change the properties of a "Shape" object with VBA.
I have a single Slide with a single Shape object,which is a table (or list) of 4 columns over 10 rows, with text in each of the cells (I don’t know if in PowerPoint, the term "cell" is adapted).
I manage to change properties of the Shape object such as Left, Top, Height, Width, but now I have to change the internal margins (or padding) in my table cells.
I guess I have to act on the "TextFrame" object and the MarginLeft property for example.
The following code is used:

Dim myShape As Shape
ActivePresentation.Slides(1).Select
Set myShape = ActivePresentation.Slides(1).Shapes(1)
      myShape.TextFrame.MarginLeft = 5

and does not work, and an error message "The specified value is out of range" appears.
Can you help me understand my mistake?

Note: I have tested the property " HasTextFrame " and the message box is displaying "0", which I guess means that the TextFrame is NULL.
But why, as the Shape contains text?

Thanks !
Platypuce