Steps to reproduce:

1. On Zabbix download site, select Zabbix version: 5.0 LTS, OS Distribution: Ubuntu, OS Version: 20:04, Zabbix Component: Server, Frontend, Agent, Database: MySQL, Web Server: Apache
2. As instructed run:
   wget https://repo.zabbix.com/zabbix/5.0/ubuntu/pool/main/z/zabbix-release/zabbix-release_5.0-1+focal_all.deb
   dpkg -i zabbix-release_5.0-1+focal_all.deb
   apt update

Results
Get:1 http://repo.zabbix.com/zabbix/5.0/ubuntu focal InRelease [4,958 B]
Ign:2 http://repo.zabbix.com/zabbix/5.0/ubuntu focal/main Sources
Hit:3 http://security.ubuntu.com/ubuntu focal-security InRelease
Hit:4 http://ca.archive.ubuntu.com/ubuntu focal InRelease
Ign:5 http://repo.zabbix.com/zabbix/5.0/ubuntu focal/main amd64 Packages
Hit:6 http://ca.archive.ubuntu.com/ubuntu focal-updates InRelease
Err:2 http://repo.zabbix.com/zabbix/5.0/ubuntu focal/main Sources
  503  Service Unavailable [IP: 178.128.6.101 80]
Get:5 http://repo.zabbix.com/zabbix/5.0/ubuntu focal/main amd64 Packages [22.1 kB]
Hit:7 http://ca.archive.ubuntu.com/ubuntu focal-backports InRelease
Fetched 4,958 B in 0s (12.7 kB/s)
Reading package lists... Done
E: Failed to fetch http://repo.zabbix.com/zabbix/5.0/ubuntu/dists/focal/main/source/Sources  503  Service Unavailable [IP: 178.128.6.101 80]
E: Some index files failed to download. They have been ignored, or old ones used instead.

I attemped to install Zabbix anyways using this command
apt install zabbix-server-mysql zabbix-frontend-php zabbix-apache-conf zabbix-agent

Returns this:
Reading package lists... Done
Building dependency tree
Reading state information... Done
E: Unable to locate package zabbix-apache-conf

Expected:
Zabbix to successfully install