Runtime error '438': Object doesn't support this property or method [...]

So no, it's not because this label has the mention "Product", it's simply because it was in the frame.

For Each Bouton_Produit In Frame_Produit.Controls 

takes all the controls in the collection, including your label.

The variable name Bouton_Produit is the name in which each element of the collection will be stored one by one.
You could have called it toto and it would have done the same thing.

Oh really?!

I was taught that I should name each button like this Button_Product1 then Button8Product2, etc!

Crazy story.

Yes, naming your buttons explicitly is good practice for your own understanding (the computer itself does not need explicit names), but here it's not really about buttons (the proof is that it takes a label).

For Each Bouton_Produit In Frame_Produit.Controls

literally means for each item in the collection of controls contained in Frame_Produits, which I put in the temporary variable (that does not exist beyond the keyword next) and without any particular type, perform the described action afterwards.
This means that any label, textbox, checkbox, listbox (in short, any control) contained in the frame would pass through this variable, and the action would be attempted (hence the bug).

You choose the name of a variable, and the name itself does not guarantee the content.