Reinstall apple fitness to watch
How to reinstall apple fitness on my Apple Watch and pair it to apple iphone?
iPhone 6s, iOS 12
Did you delete the Fitness app on the Apple Watch?
Check the apps in the Watch App on your iPhone, if the Fitness App is listed under the apps that can be installed onto the watch.
You can also download missing apps from the App Store, as explained in the last part of the article:
Download apps on your Apple Watch - Apple Support
Is the Fitness App installed on your iPhone?
Which Apple Watch are you using?