You may want to log in to your Lightsail instance as a root user. Here is how to do this on a different operating system.

Debian/Ubuntu:

a. Switch to the root user and set the root password:

sudo su -
sudo passwd root

b. Configure SSHD to allow root login.

sudo sed -i '/PermitRootLogin/s/^.*$/PermitRootLogin yes/' /etc/ssh/sshd_config
sudo sed -i '/PasswordAuthentication/s/^.*$/PasswordAuthentication yes/' /etc/ssh/sshd_config

c. Restart:

reboot

CentOS:

a. Switch to the root user and set the root password:

sudo su -
sudo passwd root

b. Configure SSHD to allow root login.

sudo sed -i '/PermitRootLogin/s/^.*$/PermitRootLogin yes/' /etc/ssh/sshd_config
sudo sed -i '/PasswordAuthentication/s/^.*$/PasswordAuthentication yes/' /etc/ssh/sshd_config

c. Restart:

reboot