Apps / Apple Watch

For what it is worth, I just tried and this same app installed just fine on my S6 here. Do you have some additional sensors that you are plugging with?

If the issue remains, I recommend contacting the app developers of Zepp Tennis directly:

How to contact an app developer - Apple Support

Try restarting both the iPhone and the watch. If that is not helping, try uninstalling and reinstalling the app on the iPhone as well.