I cannot see the actual image you are using but as a .png I do not think it is a cubemap. Secondly this is not how you apply cubemaps (or cubetextures) you are just using a standardMaterial as in
https://www.babylonjs-playground.com/index.html#LZ464X
with texture image 
Suggest you load your image onto imgur and use PG above to show your image to fully check and ask about cubetextures (of which I have little knowledge).