“does not have a Release file” for Ubuntu 20.04 upgrading nginx via add-apt-repository

I’m trying to upgrade a server from nginx 1.18 to a supported mainline version.

add-apt-repository ppa:nginx/mainline

I keep seeing an error:

E: The repository 'http://ppa.launchpad.net/nginx/mainline/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.

I’m on Ubuntu 20.04:

# lsb_release -a
No LSB modules are available.
Distributor ID: Ubuntu
Description:    Ubuntu 20.04.4 LTS
Release:    20.04
Codename:   focal

From what I’ve seen online, people have been able to upgrade their nginx to 1.23+ on Ubuntu 20.04. I checked online docs / other questions and it seems like this is all I should need to do to upgrade nginx. Is there anything else I’m missing? I tried apt-get update and apt update as well; both completed successfully.