To install synaptic:
sudo apt install synaptic
To run synaptic without root privileges:
synaptic-pkexec
Enable root privileges:
First give your linux container access to the host display. Please note that chrome OS is the host.]
xhost +si:localuser:root
If you do not want to enter the line above each time you run synaptic:
echo "xhost +si:localuser:root" >> ~/.profile
Then, you can run synaptic with sudo command:
sudo synaptic