account with that identifier already exists comes up when i try to login into my apple watch acctivity. how do i correct that?

Greetings, jthoms17!
Thanks for reaching out to the Apple Support Communities! It sounds like you’re seeing an error message when you try to log into the Activity app and I am happy to help out! First, try closing the Activity app on the Watch by following the steps in this article If an app unexpectedly quits, stops responding, or won’t open:

Apple Watch: With the app open, press and hold the side button to show the power options. Then press and hold the Digital Crown to quit the app.

Once the app is closed, reopen it on the watch to see if the message comes up again. If the issue happens again after restarting the app, follow the steps in this article to Delete built-in Apple apps on your iOS 11 device or Apple Watch and delete the Activity app and reinstall to see if the issue is still happening.
Have a great day!