I also suggest putting it in firefox if you did it in chrome... Just saying
My So couldn't register his CE Physical bonus items code for the longest time. We also bought it from the SE official store.

So we were frustrated when he got "that code has already been used" when we just unlocked the code in the store and were like wtf.

He tried in firefox instead of chrome and worked instantly.

Maybe its your browser.

If not then my suggestions are just get it from SE next time lol.