If you want to disable IPv6 on your server, below is the step to do it.

1. Edit this file:

nano /etc/sysctl.conf

2. Add these lines to the bottom of the file:

net.ipv6.conf.all.disable_ipv6 = 1
net.ipv6.conf.default.disable_ipv6 = 1
net.ipv6.conf.lo.disable_ipv6 = 1

3. Run this command on your terminal:

sudo sysctl -p

Done!