I just upgraded my old iPhone to a new iPhone and my fitness App is not working

If you are talking about the original Fitness app that requires the Apple Watch then make sure you unpair the watch from the old phone and re-pair to the new phone.

If you mean the new "watchless" Fitness app that came with iOS16 then ensure you carried over all your data using Quick Start or restore of backup and Health is turned on in iCloud settings. Settings, Your name, iCloud, Show All