https://www.techradar.com/news/windows-10-april-2018-update-is-being-blocked-on-some-alienware-laptops

As you’ve likely noticed, Microsoft recently unleashed the big Windows 10 April 2018 Update, but some Alienware notebooks are being blocked from installing the upgrade.

Reports of this issue are popping up across the internet, and Microsoft has just responded confirming that the update is blocked for certain Alienware models. Namely the Alienware 13 R3, Alienware 15 R3, Alienware 15 R4, Alienware 17 R4 and Alienware 17 R5 laptops.