If you're using the Fitness app for iPhone only (you don't have an Apple Watch,) first open the app and set it up. Then you should be able to add the widget.
This thread has been closed by the system or the community team.
You may vote for any posts you find helpful, or search the Community for additional answers.
Fitness widget
Welcome to Apple Support Community
A forum where Apple customers help each other with their products. Get started with your Apple Account.