Where is black arrow in navigation?

Did you make a course by yourself? An did you toggle the option to show notifications/directions? I have Dutch language, so I do not exactly what the right description is in English. But you need to toggle in the course planning environment.

Did you sync the course to your device from Connect? Connect should add turn-by-turn directions to your course in that case, as long as you enable the Turn Notifications toggle for the course, like DaKo said.

In your activity settings, you should ensure that Routing > Courses is set to Follow Course.

And in normal settings (hold UP from watchface), ensure Navigation > Alerts > Turn Prompts is enabled.

I have notifications to turn in my navigations, so watch alert me when I must turn and where (with white arrow on the truck). Only I don't have black arrow that appear in the bottom of the screen, under a part of maps, that show me immediatly where I must turn to the next cross.   

The white arrows on the track aren't turn-by-turn directions / turn prompts though, they just tell you what direction you're supposed to go on the course (which is helpful for things like loops where it might not be clear.) You should see those arrows on a course even if it doesn't have turn-by-turn directions built-in.

I'm talking about turn-by-turn directions / turn prompts, which, like you said, consist of an arrow that pops up at the bottom of the screen a few metres before you're actually supposed to turn.

There's actually 2 ways for you to see turn-by-turn directions:

- the notification which pops up at the bottom of the screen when necessary (which can be enabled or disabled)

- the map "data field" which appears below the map and *always* shows the distance and direction of the next turn. To turn this on, hold UP from the watchface, select Settings > Navigation > Data Screens > Map, and enable Data Field

If I were you I would turn on the map data field. If you're still not seeing the turn arrow, then it indicates either:

- a bug in the 955 firmware (that the rest of us aren't seeing)

or

- the course you use does not have turn by turn directions

Again, the way to ensure a course has turn by turn directions is to sync it from Connect (as opposed to copying the GPX file directly to the watch) and to ensure the Turn Notifications toggle is enabled.

(Another way to see turn notifications might be to set Routing > Courses to Use Map, but I think it's probably better to use the turn-by-turn directions that are baked into the course.)

Black arrow that means turn promts works in 20.29 in Bike activity. But with bugs. First, it displays too late! When I already turned. Second, too rarely. Only several turn tips was displayed during 200 km course. 

The last firmware where this feature worked normally was 17.xx version as I remember. All turns with intersections was prompted and also accordeon turns on mountain roads. And most impotant, simple black arrow that means "turn now" was displayed Before the turn!