It’s a file located in /etc folder.
/etc/fstab automounts for you everytime you boot your PC. Open it, add this line at the end and save it:
binder /dev/binderfs binder nofail 0 0
That’s because systemd-networkd should start before anbox-container-manager, but you have already started anbox-container-manager. So you should stop anbox container manager:
sudo systemctl stop anbox-container-manager
You should also enable systemd-networkd and anbox-container-manager so that you don’t need to start it everytime, by:
sudo systemctl enable systemd-networkd anbox-container-manager