Depending on what Raspbian version you are running on your Pi, the approach for enabling a serial tty login via a VT terminal differs, but on current/recent versions you can enable by enabling and starting this systemd service (steps from this post, and here):
sudo systemctl enable serial-getty@ttyUSB0.service
sudo systemctl start serial-getty@ttyUSB0.service
This assumes you are using a USB serial dongle and that it’s connected as /dev/ttyUSB0. You can check by doing a ‘ls /dev/ttyUSB*’ before you connect your USB serial adapter and after to check what device your USB dongle appears as.
If you’re running an older version of Raspbian not using systemd, you can add a line to /etc/inittab to initialize getty as described here.