Activity App not working on iPhone X

Hi

The Activity app only becomes available on your iPhone when you are using an Apple Watch:

• Set up your Apple Watch - Apple Support
• Track daily activity
• Set up your Apple Watch - Apple Support

The Health app on your iPhone otherwise tracks health and activity information:

Use the Health app on your iPhone or iPod touch - Apple Support

Did you checked your health app ?

Maybe your watch is not synced mode in health app.

If there is two more Watches are listed -or- you have removed the correct watch in the device list

Your may need to reset and sync your watch to listed