You've paired your Stryd with the Apple Watch, but HealthFit isn't transmitting Stryd metrics to Tredict?
The problem here is the lack of support on the side of the Apple Health API.
HealthFit obtains the activity data of the Apple Watch from the "Apple Health" interface and can therefore only transfer the data available there.

How do I synchronize my Apple Watch with Stryd activities to Tredict?

Unfortunately, we can't offer automatic synchronization when you use an Apple Watch together with a Stryd, yet.
As an Apple Watch user, you can download your activities from the Stryd Powercenter and then manually upload them to Tredict.

Pair and sync the Stryd with devices from other manufacturers?

If you use your Stryd together with a Garmin, Polar, Suunto or Coros device, you don't have to worry.
The syncronization works seamlessly and automatically via the corresponding interface of the device manufacturer.