Why i can’t get the fitness app to work

Other than asking why it's not working, you've given very little information about what isn't working. See this link and see if there is anything which might help you --> Use the Workout app on your Apple Watch - Apple Support

I'm not sure why it's not working for you. You can use the Support button at the bottom left of this page, or schedule an appointment at your Apple Store Genius Bar and take your watch and iPhone with you, or call Apple (if in the US) at 800-MY-APPLE (though if you don't have an AppleCare+ Plan, phone support may not be free).

Are you trying to use Fitness or Fitness Plus? Have you downloaded the Fitness App to your iPhone?

Is it downloaded to your iPhone?

You're welcome. I'm sure Apple will get it sorted for you.