• login as root ssh root@SERVER_IP
• check if sudo is installed by sudo
• if not
  • enable the su-mode su -
  • install sudo apt install sudo -y
  • check with visudo if the file contains these two lines, if not, add them

    # Allow members of group sudo to execute any command
    %sudo   ALL=(ALL:ALL) ALL

• add user with adduser USERNAME
• give sudo power to the user usermod -aG sudo USERNAME
• you can login as a user ssh USERNAME@SERVER_IP

Resources:

➡️https://www.digitalocean.com/community/tutorials/initial-server-setup-with-debian-9

➡️https://unix.stackexchange.com/questions/354928/bash-sudo-command-not-found