Hello cmw1978,
We understand that you're looking to use the Activity app on your iPhone along with your Apple Watch, and that's a great way to track your workout activities. You've come to the right place for assistance with this.
To get started, which exact iOS version did you recently install on your iPhone?
Also, which version of watchOS is currently installed on the Apple Watch?
We'd recommend checking out the following resource to check for any profiles installed on your iPhone to see if this may be related to what's preventing the app from being installed on your iPhone:
How to delete an app that has a configuration profile on your iPhone, iPad, or iPod touch
Specifically the second step of the section labeled "Delete the app and its configuration profile."
Have a great evening.