Windows 10 May 2019 Update will be blocked from upgrading in the following situations:
The computer is connected to a USB storage device or SD memory card
Microsoft said that if your PC uses a USB storage device or SD memory card, you can't upgrade to the Windows 10 May 2019 update. A simple solution can solve this problem, which is to unplug it. Microsoft has recommended that users remove any external USB storage devices and/or SD memory cards to initiate the upgrade installation process.
Older versions of anti-cheat software
If you use an older version of anti-cheat software bundled with many popular games, you may not be able to install the Windows 10 May 2019 update. Microsoft has found a bug that old versions of anti-cheat software may cause Windows 10 to update PC crashes in May. Most games have been updated for this issue and Microsoft is actively working with affected partners. If your Windows 10 May 2019 update is blocked, make sure you are running the latest version of the game.
Known folder isRedirect
Microsoft also prevents Windows 10 May 2019 updates from being installed on devices with any known folders. If you have redirected any known folders (eg desktops, documents, images), they will be in the %userprofile% directory when updating. Create an empty folder with the same name. If you choose to back up your content to OneDrive using the OneDrive wizard, or if you choose to back up your content during the Windows Out of Box Experience (OOBE), your known folders will be redirected. You may also have manually redirected Known Folders through the Properties dialog in File Explorer. This issue does not result in the deletion of any user files and the issue is being fixed.
Microsoft is expected to resolve all of these issues before pushing to more users at the end of May, but if the updates on your device are blocked, make sure you meet the above requirements.