how to install sudo:


$ su
$ apt update
$ apt upgrade
$ apt install sudo
$ sudo usermod -aG sudo user_name
$ systemctl reboot

after reboot you can use a sudo


or


$ su
$ apt update
$ apt upgrade
$ apt install sudo
$ nano /etc/sudoers

under the line "root ALL=(ALL) ALL"
add your user:
user_name ALL=(ALL) ALL

ctr+x - save & exit

$ exit

now you can use a sudo