How to cancel iCloud Subscription successfully?

I try to cancel my Icloud subscription through the system preferences/icloud storage/manage/change storage plan/downgrade options. Then I receive a question to fill in my password. After doing this, probably 2000 times, I receive a notification:"you can't manage your storage at this time. Wait a moment and then try again". It looks like "mission impossible" to cancel a subscription! The reason is because Apple withdraw money from my credit card once a week instead once a month for the same service. Everybody should be able to stop paying for a service at anytime, right?

[Re-Titled by Moderator]