According to your account you are trying to send Courses to the device rather than Workouts. The Courses must be created manually in the Course widget or saved from activity you previously uploaded. Of the 3 Courses in your Sync menu, only one of them is saved. I suggest you clear all the items in your Sync menu then review the items in the Course menu. Find the two activities that are not saved then open each activity and save the activity as a Course. Once you have them all in the Course menu you can send them to the device. Since there are only 3 I would send them to the device one at a time.
How do I create a course? See the last section on creating from a previous activity.
I'm actually trying to send an activity to my Fenix 3 that I recorded with a Suunto so that I can "race" it using the virtual partner feature. I uploaded the gpx file and then clicked on the activity in garmin connect and hit send to device and get the error. Should I do it a different way?
I also tried saving it as a course like you suggested and then sent to the Fenix but garmin express is hung at syncing...going on 10 minutes now.
The Course will have to be created manually in the Course widget to send it from Connect. Unfortunately, Courses created from a .gpx file will not sync. The Connect Team is aware of this issue and it will be resolved in a future version of Connect.
You do have a possible work around with the fenix3 that other fitness devices don't. Since the fenix3 is part outdoor device you can use BaseCamp to read the track in the .gpx file and send it to the fenix3. The data in the .gpx file will show up in BaseCamp as a track, just locate the track in the My Collection folder and send it to the fenix3. Eject the fenix3 from BaseCamp and disconnect it. The track will be converted into a Course and show up in the Courses menu on the watch. Please understand this is a possible work around in an emergency and I'm assuming you are fairly comfortable with computers, applications, and their user interfaces. If this doesn't work, creating the Course manually in Connect is the only option. A Course created in Connect doesn't contain GPS data so it can be used for Navigation. In order to use the Race an Activity feature, the source of the Course has to contain GPS data.
