HRV Stress Test not accesible in 955 why?

Because of the product segmentation. All want is "plastic" fenix. All the fenix functionality / software team and weight of forerunner. Garmin you can call it Fenix SE, this one is for free :)

They sell hardware, but they act like they are selling software only.

Garmin customers can't just choose the watch model suits him best and enjoy the platform. 

Garmin customer must dig around and find out what software each watch offers (all in the same generation) and buy that watch, even if he doesn't like the looks, size or weight. 

It's a big mess!

Is this not similar to the Health Snapshot that is on the newer FW versions?

That’s my thought too.

No, It is a test that you can do in every moment or condition. The Health Snapshot is a mean  along night hours only

Not correct , you can do a health snapshot at any time , that is what it is meant for , not HRV overnight recording.

It an (Activity/App) on the watch, like where RUN etc is 

It saves a snapshot of HRV Stress etc at a particular time

Here is the document on it

support.garmin.com/.../

Yes, but I want to get the graphic of HRV and with this Snapshot you havn't data on It even you use a HR band or don't.HRV Stress Test is the only method to get data on It.

Is you watch new?It takes quite a while for HRV to get trended. That is the most reliable way to do it and you get a better trend like that. One snapshot spike at a given time wont give you the good information and a building up trendline. It works very well and is quite good at picking up when something is affecting you. The other day I got sick and it pick it up the night before as my HRV when much lower then before.

See support doc below.

support.garmin.com/.../

The HRV Stress Test is more focused on training readiness when you run the test. 

It requires a chest strap.

Anyway, for those with chest strap, there are better smartphone apps to run HRV tests.