Back in November last year, Microsoft and Apple together blocked iCloud from installing on Windows 10 October 2018 update and vice versa due to an incompatibility which caused issues updating or syncing Shared Albums in iCloud. Apple later released an updated version of iCloud (7.8.1) to fix this compatibility issue.
Microsoft has now announced that the upgrade block will be removed mid-February, after which these blocked devices will be offered Windows 10, version 1809 automatically.
If you are running an older version of iCloud, Apple or Microsoft may notify you to update your iCloud for Windows to version 7.8.1 before attempting to upgrade to Windows 10, version 1809. You can download this latest version of iCloud for Windows here.