I'm having the same issue. I have no clue what Error Code: 107-3021 is.
I've tried almost everything and I still can't purchase anything from Nintendo eShop not even download the free Mercedes-Benz DLC for MarioKart 8.
Even in Smash Bros.WiiU i cant download Mewtwo as a DLC character without the error code 107-3021 showing up again.
I can do everything online such as playing online with friends and using the internet but I just can't purchase any DLCs.
it also says "information help: 1. 5.3.2U // 2. WUP-001(02) // 3. FW403504518 // 4. AMK4-0210-7375.
WHAT IN THE WORLD THIS MEANS?!?!?!
The worst part about this is Nintendo support site dont even know what this code is. When I type it in the support site it says the code is unrecognizable.
PLEASE IF ANYONE HAVE ANSWERS FOR THIS THEN LET US KNOW!!!