No possibility to downgrade iCloud storage plan

I wanted to downgrade my iCloud, because I want to stop use my Mac. Already removed all the files from iCloud, there's around 300MB leftovers. When I go to Settings > my user > iCloud and I want to downgrade. I constantly receive following message:

You cannot manage your storage right now. Please try again later.

I went through Apple ID password reset to make sure that everything is ok, and nothing helps. I'm lost. I even installed iCloud on Windows, logged in successfully, and when I try to do the same with that app, I receive "Access denied to buy.itunes.apple.com". Can anyone help me?