I know that the pibrella board is not sold anymore, but I still have one and just needed the buzzer for a quick test.
In the node-red-node-pibrella doc I can read the following:
## Pre-reqs
Requires the RPi.PIO python library version 0.5.8 (or better) in order to work. See the [RPi.GPIO site](https://pypi.python.org/pypi/RPi.GPIO) for mode details.
sudo apt-get -y install python-rpi.gpio
but my Raspian Bullseye tells me:
Package python-rpi.gpio is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
Nevertheless I installed the node, and the Palette says:
Warning : Can't find RPi.GPIO python library.
How can I install a RPi.GPIO python library for Bullseye?
That node is liable to very old and prob rely on Python2
[edit] I just looked at the pibrella node and it is maintained by Nick and Dave so maybe it might get updated to work nowadays[/edit]
Workaround could be to just use the standard Pi output node to make it beep
This works for me on one of mine
It's working!
However, I have to use a fixed frequency. Can I send the frequency with the payload, so that I can change the frequency by script or via slider?
I don't think the basic node allows that unfortunately 
Just tried the pibrella node out on my bullseye and I get the same error message in the console as you which is strange as RPi.GPIO is definitely installed
I looked at code in the node and it's checking for old python RPi.GPIO library
I changed line15 in
[edit - corrected file name 
]
/home/pi/.node-red/node_modules/node-red-node-pibrella/38-rpi-pibrella.js
if (!fs.existsSync("/usr/share/doc/python3-rpi.gpio")) {
and it now works for me
I'll issue a PR for it
You changed line 15 in the .js file in this directory.
Is it always 38-rpi-pibrella.js?
After installing the node again and changing the above check for the gpio file, I can use the pibrella node.
Thank you!!
Sorry to have missed out he vital bit of the filename
My PR has been accepted and merged so the fix should be available in the next NR release
