Magento 2.2.1 - Possible upgrade issues

The issue of availability of the module has been resolved by Magento. Please check upgrade Magento to version 2.2.1 now.

composer require magento/product-community-edition 2.2.1 --no-update
composer update
php bin/magento setup:upgrade
php bin/magento cache:clean
php bin/magento cache:flush

As Piotr commented to the Github issue linked in the comments above, the problem is a caching/permission issue.

See here:

We have some caching issue on our side and not all accounts were updated with permissions for the new release or signifyd module. We are working to find and eliminate the root cause but in the meantime:

• you can try to generate a new set of keys on Magento.com ->My Account -> Marketplace. Did not test it but it might work. And use those keys in auth.json
• you can DM me (eg on Twitter) or email (peter(at)magento.com) with your pubkey or MAGEID and someone will help clear the cache (it is per account, not global)

Workaround if you don't need the signifyd module: Add it to your "replace" section in the composer json as follows:

"replace": {
        "magento/module-signifyd": "*"
    },

This is the best way to remove unneeded modules. If you don't need that module as well, I'd leave that change in your composer.json. The composer.json is made for modification, your changes can stay. If you need the signifyd module, you should revert the change later of course.

Source: https://github.com/magento/magento2/issues/12089#issuecomment-342733758