ProxyChanger APT module tries to write proxy configuration in the file /etc/apt/apt.conf.d/90proxy. So, to be able to update the contents of this file, the current user has to have write permissions. There are multiple ways to do this, depending, for example, if the computer is used by multiple users or not, so permissions can be given to a single user or a group.

To keep things simple, we can run this commands to allow the current user to write that file:

sudo touch /etc/apt/apt.conf.d/90proxy
sudo chown ${USER} /etc/apt/apt.conf.d/90proxy
sudo chmod u+rw /etc/apt/apt.conf.d/90proxy