dnsmasq: failed to create listening socket for port 53: Address already in use

Check what's listening on port 53 (domain) with:

sudo ss -lp "sport = :domain"

Disable any service that is running on this port. It's usually systemd-resolved

Here I make sure that you have stopped the systemd-resolved service. I'm going to also mask it so it doesn't auto start on reboot.

sudo systemctl stop systemd-resolved
sudo systemctl disable systemd-resolved
sudo systemctl mask systemd-resolved

To undo what you did:

sudo systemctl unmask systemd-resolved
sudo systemctl enable systemd-resolved
sudo systemctl start systemd-resolved