Error: ImportError: No module named pyrealsense2

I followed all instructions and read and tried everything that has already been written about it in the forum.
But I just can't get any further.
I'm almost desperate.

I would like to test the rs-hi-realsense example.

But when importing pyrealsense2 this error always comes up, although I have already tried everything.

I just don't understand where the mistake is.
I am using RaspberryPi4, with Debian Buster.

I use this example here, which I found on the Internet:

import pyrealsense2 as rs

while True:

       frames = pipeline.wait_for_frames()
      depth_frame = frames.get_depth_frame()
      if not depth_frame: continue
      width = depth_frame.get_width()
      height = depth_frame.get_height()

#print(width,height)

#Calculate distance

dist_to_center = depth_frame.get_distance(int(width/2), int(height/2))
print('The camera is facing an object:',dist_to_center,'meters away')

Many thanks for your help.
I'm really grateful for a helpful tip.

Greetings

Martin