Cannot reduce my iCloud monthly package

I am using a MacBook Air, IOS X, and recently purchased 1TB for $19.99 a month. I now want to reduce to the free 5G and when I click Downgrade Options I am requested to supply my password. When I put in my password and hit Manage the screen returns to the state of entering the password. (without additional message like incorrect password or anything!). I would appreciate any help in reducing my account!