sysctl
Do not use the sysctlmethod. Other tutorials show it and IPv6 will just re-enable itself after packages update.
GRUB
sudo nano /etc/default/grub
Locate this like GRUB_CMDLINE_LINUX_DEFAULT="quiet splash.
Add ipv6.disable=1.
Now, it will look like this GRUB_CMDLINE_LINUX_DEFAULT="quiet splash ipv6.disable=1
Press Ctrl + O then Enter
Run sudo update-grub
Restart your PC.