Can't add ppa to Focal or Ubuntu 20.04

SunBear ; Hello

The advisory is a true fact "does not have a Release file."
The package last seen some love >> zesty/ 2018-05-09 09:17
see: http://ppa.launchpad.net/bluetooth/bluez/ubuntu/dists/

It has been moved to the software repository.
Have your tried the bluez package that is available in the repo ? >> bluez/focal-security,focal-updates 5.53-0ubuntu3.4 amd64

You do need to remove the PPA source - and maybe purge the PPA version to install the repo version.

-hope this helps-

Install through your repository.
PPA installs are: End users install PPAs at their own risk.
Bugs may be involved.
https://help.ubuntu.com/community/PPA
This the repository package:
https://packages.ubuntu.com/focal/bluez
FACT.. @https://launchpad.net/~andrew-woodhead666
You can install this through command line, or install through the Synaptic Package Manager.
Question: Did you install your OS from a daily build from Ubuntu 20.04 ?
Every OS that has been released, has a release file. Quote : "focal Release' does not have a Release file."
@https://launchpad.net/~m-hampl
?..

The PPA doesn't support your release of Ubuntu and you should remove it

Which package do you want to install, bluez or bluez-btsco?
https://packages.ubuntu.com/focal-updates/bluez
https://packages.ubuntu.com/focal/bluez-btsco

$ dpkg -l | grep bluez
ii bluez 5.53-0ubuntu3.4 amd64 Bluetooth tools and daemons
ii bluez-cups 5.53-0ubuntu3.4 amd64 Bluetooth printer driver for CUPS
ii bluez-obexd 5.53-0ubuntu3.4 amd64 bluez obex daemon

The above bluez pkgs are in Ubuntu 20.04. However, they don't seem to be able to connect with an OneOdio Pro C Bluetooth headphone. I was hoping to use the ppa to get the latest version.

There may be other PPAs than the one that you tried, see https://launchpad.net/ubuntu/+ppas?name_filter=bluez for a list of candidates (but maybe none of them contains a higher version for focal).

For diagnostic purposes you might run the https://help.ubuntu.com/community/WirelessTroubleshootingProcedure (does not only check WLAN, but also some bluetooth functions).

There is a higher version of the bluetooth packages in Ubuntu jammy (22.04, the development version). You could download the daily build https://cdimage.ubuntu.com/daily-live/current/ and run it in the "try Ubuntu without installing" mode to test whether the version in jammy (currently 5.63) works for you.

Remark to https://askubuntu.com/questions/1382544/how-to-pair-with-or-connect-to-an-oneodio-pro-c-wireless-headphone-on-ubuntu-20

As far as I can see, the Archlinux package pulseaudio-alsa contains just a single file /etc/alsa/conf.d/99-pulseaudio-default.conf with the contents as shown in https://github.com/archlinux/svntogit-packages/blob/master/pulseaudio-alsa/trunk/asound.conf

I do not think that this will help if pairing your device fails.