Can I install this product in the walls too?

Yes, but only if there is an air gap in the assembly; typically the air gap would be between the outer layer and the foil layer. If the building is conditioned, you’ll use the foil in addition to the regular insulation.
If you add the radiant barrier on any external walls that are getting direct sunlight, this will make a big impact on the temperature in the areas sharing those walls. The best way to install it is to have the foil closest to the outside layer as possible, while still maintaining an air space on one side of the foil.