A couple of days ago, Windows 10 v1903 was released to the public. I downloaded and installed it on a thumb drive. No problem there.
I have tried to upgrade two machines running Windows 10 Pro. Neither would take it. The upgrade process now has a new screen that says that my PC isn’t ready for this version.
I found an article a few minutes ago saying that they will be pushing out the update in June. It may be that have it blocked until then.