On the server (debian) I had to enter after the command from the instructions:
sudo apget install python3-pip
additional commands:
sudo apt-get install build-essential libssl-dev libffi-dev python-dev
sudo pip3 install cryptography
sudo pip3 install paramiko
sudo pip3 install pyasn1
I just managed to run the script properly and install all the plugins:
sudo python3 install_plugin.py
Plugins have installed correctly and there are already corresponding files in the paths (as on the old 2.5 server):
/etc/ocsinventory-server/plugins
and
/etc/ocsinventory-server/perl/Apache/Ocsinventory/Plugins
Unfortunately, the problem occurs all the time and data from plugins from client stations do not save on the server (i can't see them in ocs-reports)