This worked for me, my guess is it’s a SKU that’s designed for bundling with another product (e.g. buy an iPhone and get four months of Apple Music free).
@yochai, did you get that far? I’m not really interested in this subscription so I haven’t confirmed.
Yes. After that I got the error.
It looks like there’s a (lifetime?) limit on how many times you can use these codes, even for existing accounts. I think I used the free codes twice.