Can't add repo to install on Rpi

Trying to install matrix software on Rpi3 running Buster OS. Your directions for setup are not working.

curl | sudo apt-key add -

gives this error:

curl failed to verify the legitimacy of the server and therefore could not establish a secure connection to it.
How to fix this?

The repo has been moved to s3, this url should work:

Thanks, that does work. Confusingly, the key file is still in place at the old url, so that it gives an expired certificate error rather than a file not found error. Now, if only the webmasters at the matrix and hackster sites, all of which give the erroneous link, knew of this important change. Or if the “dev team”, to which corporate headquarters directed my email inquiries, knew about it.

For anyone else needing it, here is the correct sequence of commands to load the matrix kernel and software onto the pi and prepare the matrix for re-flashing:

curl | sudo apt-key add -
echo "deb $(lsb_release -sc) main" | sudo tee /etc/apt/sources.list.d/matrixlabs.list
sudo apt-get update
sudo apt-get upgrade
sudo apt install matrixio-kernel-modules
sudo reboot
sudo apt install matrixio-creator-init
sudo reboot
sudo voice_esp32_enable --chip esp32 --port /dev/ttyS0 --baud 115200 --before default_reset --after hard_reset erase_flash

I am having this problem again… or still.

yeah - I get:
apt-get update
Hit:2 Index of /raspbian bullseye InRelease
Hit:3 Index of /debian bullseye InRelease
Ign:1 bullseye InRelease
Err:4 bullseye Release
404 Not Found [IP: 443]
Reading package lists… Done
E: The repository ‘ bullseye Release’ does not have a Release file.

Will Matrix Voice be supported on Debian/Raspian Bullseye?
When to be expected?
Any alternatives?