If your watchOS version is not supported, you can not install it on your Apple Watch.
Please update your Apple Watch to the latest version of watchOS.
👉Apple Support: Update your Apple Watch
Zones ver 3.0 requires watchOS 5.1 or later.
Apple Watch 1st generation (a.k.a Series 0) cannot install Zones app because the final OS is watchOS 4.
If your Apple Watch is Series 1 or later, it is compatible with the latest watchOS. Please update watchOS to the latest version, and install the app to your watch.
If you have apps you can install on Apple Watch, they'll be listed at the bottom of the screen under the Available Apps header.
👉Apple Support: Use apps on your Apple Watch
If you know you've downloaded an iPhone app that has a watch component but it's not showing up under the "Available Apps" banner, chances are you have Automatic App Install turned off and your iPhone didn't initially recognize that these apps were Apple Watch compatible. As such, your iPhone may need a reboot to properly sync its app collection.
👉Apple Support: Restart your iPhone
Please check your Bluetooth connection on both the iPhone and Apple Watch.
If toggling Bluetooth on and off didn't work, you may want to try force quitting the Watch app on your iPhone.
Try a restart for either your iPhone or Apple Watch, then re-open the Watch app.