I can select the mirrored face, by checking the "wrap to face" option. However, this isn't the issue. You need to flip the normal plane of Workplane 4. Select the workplane 4, right-click, select "flip normal". This will (should) flip the image "upside down" relative to the other image. Edit the sketch to rotate the image so it's oriented as desired, finish the sketch and execute the decal command again. It will work. Currently, the sketch plane is normal towards the inside of the part, so the decal isn't visible on the outside face. Flipping the normal plane so it's towards the outside of the part fixes this, and also allows you to select the face you want. I don't know, but I suspect that - buried deep inside the decal command - Inventor sees that you are trying to place a decal on the inside face, rather than the outside face, if that makes sense. It might even be working, you just cant see it! 😉