https://www.techradar.com/news/own-an-original-apple-watch-you-wont-be-able-to-upgrade-to-watchos-5

Apple has just announced watchOS 5, and has confirmed that if you own the original Apple Watch you won't be able to download the new software when it's released later this year.

Only the Apple Watch Series 1 and above will be able to download the next-gen software when it's released later in 2018.