I have tested it with a fresh flashed Raspbian Buster Light on a RPi 4B:
$ sudo apt-get install bind9 bind9utils dnsutils
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following additional packages will be installed:
dns-root-data libirs161 python3-ply
Suggested packages:
bind9-doc ufw rblcheck python-ply-doc
The following NEW packages will be installed:
bind9 bind9utils dns-root-data dnsutils libirs161 python3-ply
0 upgraded, 6 newly installed, 0 to remove and 0 not upgraded.
Need to get 1,675 kB of archives.
After this operation, 4,605 kB of additional disk space will be used.
Do you want to continue? [Y/n] Y
Get:1 http://raspbian.raspberrypi.org/raspbian buster/main armhf libirs161 armhf 1:9.11.5.P4+dfsg-5.1 [235 kB]
Get:2 http://raspbian.raspberrypi.org/raspbian buster/main armhf dnsutils armhf 1:9.11.5.P4+dfsg-5.1 [361 kB]
Get:3 http://raspbian.raspberrypi.org/raspbian buster/main armhf python3-ply all 3.11-3 [65.1 kB]
Get:4 http://raspbian.raspberrypi.org/raspbian buster/main armhf bind9utils armhf 1:9.11.5.P4+dfsg-5.1 [422 kB]
Get:5 http://raspbian.raspberrypi.org/raspbian buster/main armhf dns-root-data all 2019031302 [5,396 B]
Get:6 http://raspbian.raspberrypi.org/raspbian buster/main armhf bind9 armhf 1:9.11.5.P4+dfsg-5.1 [587 kB]
Fetched 1,675 kB in 1s (1,964 kB/s)
Preconfiguring packages ...
---snip---
So you have to look at your setup, first at /etc/apt/sources.list
, or use a fresh flashed image. You may also try to reinitialze the package lists. How to do it you can look at Raspberry Pi sudo apt-get update not working.
apt policy bind9
on my Pi4 says it's available. Please add the full error message to your question. – Dirk Jul 24 '19 at 20:18