HRV Stress App

It is entirely possible that your Garmin watch cannot read the HRV data from the Wahoo Tickr. And since the HRV data is needed for the GRV stress test, that could be why.

A couple of years ago I used to own a Tickr X HRM (bought it for the built-in memory, this was before HRM-Tri). Mine did not support HRV, but perhaps newer firmware has fixed that in the mean time. Do you get HRV data when you use the TickrX during activities like running/biking? If not, maybe it is possible to update the Tickr X

Hmmm, I don't know how to tell.  I get HRV data no problem with two other activities from Connect IQ store but, these days, so many apps 'fake' the data--they estimate it from something rather than measure directly. There are many HRV measurements that rely on different phases of the heart beat and different mathematical calculations and those other apps give me all of them without any issues.

What makes me think that HRV data is possible to collect with Wahoo TickrX is that HRV STRESS test itself was successful twice.  The rep on the phone focused on standing still but I never got it to work while standing.  I got the test to work while sitting once and while in a lazy chair after a ride another time.  According to the rep, being still is very important.

HRV is also not something that needs to be calculated in the HRM directly.  It's the cumulative measurement based on many successive heart beats or the differences in the times between two beats to be precise.  Seems to me that any device that is capable of measuring Heart Rate can be used to calculate how different the timing of each beat is. 

Edit: clarification about sampling

If it has worked before, the Wahoo is clearly capable of providing the right data, so it must be something else. Changing the battery is a bit of a lame advice, but just maybe if you haven't already tried that..

As to HRV support; I now realize device software has of course also changed. The Fenix 3 refused to (re-)calculate HRV based on the 1 or 5 second-interval (averaged BPM data per 1 or 5 second interval, ie 60 or 12 per sampling intervals per minute. I don't remember if it was 60 or 12 per minute) that the Tickr X sent. HRV data just wasn't present in the FIT files.

1-5 seconds is too short of a timeframe to estimate HRV.  Depending on the HR you have at the moment, there maybe only a couple of beats or even none in the one second interval and you need at least 30 to calculate anything meaningful.  Thirty beats can take a full minute depending on your fitness level.  I wonder if there was an option on your Fenix 3 to increase the interval length.  Note that the current HRV STRESS test requires 3 minutes of measurements.  The other apps I was using require 5 minutes.  That's way more than 5 seconds.