Microsoft has resolved a known issue that was blocking customers from upgrading some devices running Windows 10 1903 and 1909 to newer OS versions. The known issue affects both client (Windows 10, version 1909 and Windows Server, version 1903) platforms. KB4579919 is now available through the Windows Update service, to be downloaded and installed automatically on all affected systems. Customers who cannot immediately apply this update are advised by Microsoft to work around this known issue by re-enabling HTTP access to the Internet to the Windows 10 Setup Dynamic Update.
Source: https://www.bleepingcomputer.com/news/microsoft/microsoft-fixes-known-issue-blocking-windows-10-upgrades/