Q: I'm unable to downgrade from 200GB to 50GB

I didn't see the 50GB as an option when I purchased the 200GB...now that I realize I don't need THAT much storage, I'd like to downgrade my plan but when I click on the 50GB option, the "DONE" button grays out and doesn't allow me to submit my choice and continue.

Any ideas how to downgrade? I've tried both on my iphone and on my mac through icloud.com

Thanks!