Is your Fenix 5 updated to the latest software (current release is 7.10)? The function for Strength Training workouts was only introduced during a software update released a couple of months ago, so if you’re still rolling on an old version it’s time to update.
If you are already using the latest software or are still having problems after updating then are you trying to sync the workouts using the mobile app? There is currently a bug in the mobile app that doesn't allow you to send workouts to devices. Garmin has posted in threads discussing it that they are aware of it and are working on it, so hopefully a fix is coming soon. In the meantime the best way to get around this is to use the USB cable and Garmin Express on a PC or Mac to sync workouts. That still works just fine.
But?!?!?!? Just to understand :D I love Garmin from an hardware point of you... but are they able to develop software and firmware? I have an Edge 1030, and Edge 820, a Fenix 3 HR and now a Fenix 5, and Edge Remote and some other sensor, and I continue to follow this forum because they are full of software issues!!! Damn it! They are so weak from a software point of you! It's incredible, something to be shamed about o.O
