Triathlon Tracker requires watchOS 4.1 or later. Please follow the steps here to make sure your Apple Watch's software is up-to-date.
If your watchOS is up to date, please check in the Watch app on your paired iPhone to make sure that "Show App on Apple Watch" is enabled. To do this, open the Watch app, find Tri Tracker, tap on it, and make sure the setting is enabled. It looks like this:
If none of the above works, try restarting your Apple Watch.