Zabbix installation repository does not have a release file?

I am attempting to install the Zabbix monitoring software on an Ubuntu 20.04 server.

I am following the instructions here: https://www.zabbix.com/download?zabbix=6.0&os_distribution=ubuntu&os_version=20.04_focal&db=mysql&ws=apache

When I try to update, I get this error:

user@someserver:~$ sudo apt-get update
[sudo] password for user: 
Hit:1 http://archive.ubuntu.com/ubuntu focal InRelease                                               
Get:2 http://archive.ubuntu.com/ubuntu focal-updates InRelease [114 kB]
Get:3 http://archive.ubuntu.com/ubuntu focal-backports InRelease [108 kB]                      
Get:4 http://archive.ubuntu.com/ubuntu focal-security InRelease [114 kB]                       
Get:5 http://archive.ubuntu.com/ubuntu focal-updates/main amd64 c-n-f Metadata [14.7 kB]       
Get:6 http://archive.ubuntu.com/ubuntu focal-security/main amd64 c-n-f Metadata [9,564 B]            
Ign:7 https://repo.zabbix.com/zabbix/5.4/ubuntu focal InRelease                                      
Err:8 https://repo.zabbix.com/zabbix/5.4/ubuntu focal Release 
  Could not handshake: Error in the pull function. [IP: 178.128.6.101 443]
Reading package lists... Done
E: The repository 'https://repo.zabbix.com/zabbix/5.4/ubuntu focal Release' does not have a Release file.
N: Updating from such a repository can't be done securely, and is therefore disabled by default.
N: See apt-secure(8) manpage for repository creation and user configuration details.

This causes the whole installation to break, as I can pull some (most?) needed files and executables directly from the Ubuntu Focal repos, but there are a couple files Zabbix needs that seem to only be available from them.

Does anyone have some advice? Thanks.