Enable Serial Terminal on Debain using Grub Configuration (Grub2)

Edit /etc/default/grub, enter:

# vi /etc/default/grub

Append / modify as follows:

GRUB_CMDLINE_LINUX='console=tty0 console=ttyS0,19200n8'
GRUB_TERMINAL=serial
GRUB_SERIAL_COMMAND="serial --speed=19200 --unit=0 --word=8 --parity=no --stop=1"

Save and close the file. Next run the following command to generate a grub2 config file /boot/grub/grub.cfg :

# update-grub

Ref:
https://www.cyberciti.biz/faq/howto-setup-serial-console-on-debian-linux/