Alright I see the issue. Which is images are added as path instead of just a file name. The extension will correctly load the cubemap if the images are in the root folder of the project which happens on default if you add image by choosing "File(s) from your device".
But as a user you can't know that, also Gdevelop grows so fast and it has this kind of strange quirks which wasn't like this when this extension created.
I worked on the extension code and I fixed that issue. Now it doesn't matter if the image is added as a path or just shown as image file.
I also noticed another issue related to having multiple 3D layers in scene (such as having a Gun in another 3D layer) So I added a Layer selection property to the action. It should be leave on "Base layer" in many cases. But now targeting the spessific layer fixes the overlapping environment maps for each 3D layer.
Expect to have the updated version of the extension dropped in 5-10 minutes.
Also Gdevelop built-in skybox uses non standard orientation for its cubemap. Please for this extension use the Z+ up orientation (middle option) from that web tool. which is a standard orientation and this extension follows that standard.