It's not working yet, but your suggestions were somewhat helpful, Akkana. 
I got this result:
ll@leno:~$ sudo apt install yt-dlp
...
yt-dlp is already the newest version (2023.03.04-1~bpo11+1).

That seems to contradict the other output:
Available version: stable@2023.07.06, Current version: stable@2023.03.04

Even though it didn't solve this issue, it's helpful to know this basic approach. I played with it a bit. I removed yt-dlp and installed it again. Being the older version, it didn't update, but it got me a new bit of basic knowledge. So, thanks to Akkana for that, even though the initial issue remains.

I'm guessing that there is a newer version, which is necessary to have, but that the newest version is not available in the apt repository.
This refers to John's postulate: "But it looks like the second machine used 'apt' to install the package, while the first one was installed by another means?"

I don't recall how yt-dlp was installed on the len4 machine, where the update was successful, but I have a vague memory that youtube-dl (and now yt-dlp) was known to have older versions in apt, which made it advantageous to install it from a different source. I don't recall what that source is or how to do that.

Anyone know that?

In gratitude,
LeRoy

-- 
There is something glorious birthing within all of us.
I am the Love of God, no matter what.
BSA
LeRoy Diener

213-LEROYIZ
213-537-6949
https://leroydiener.com

On Tue, 2023-08-29 at 14:28 -0600, Akkana Peck wrote:
I'd do what the message says, and use apt to update it.

You should already be running sudo apt update && sudo apt upgrade (or full-upgrade), which will pull in new versions of yt-dlp, but if you want to do just that package, you can
sudo apt install yt-dlp

Or if the apt package isn't new enough, you can
sudo apt remove yt-dlp
and then install the other version.

        ...Akkana

LeRoy Diener writes:
Hi NMGLUG folks,

Here's an interesting error that I'm trying to resolve.

On one computer, len4, with Debian 11, I updated yt-dlp successfully,
as shown here
ll@len4:~$ sudo yt-dlp -U
[sudo] password for ll:
Available version: stable@2023.07.06, Current
version: stable@2023.03.04
Current Build Hash: 
91cad9f121c1f6f0a81b747415c46ecba0ff331ed38cc6433040b4ac7b6e15ca
Updating to stable@2023.07.06 ...
Updated yt-dlp to stable@2023.07.06
ll@len4:~$

On another computer, leno, also with Debian 11, I get an error trying
to update yt-dlp, as below
ll@leno:~$ sudo yt-dlp -U
[sudo] password for ll:
Available version: stable@2023.07.06, Current version:
stable@2023.03.04
ERROR: As yt-dlp has been installed via apt, you should use that to
update. If you're on a stable release, also check backports.
ll@leno:~$ lsb_release -a
No LSB modules are available.
Distributor ID: Debian
Description: Debian GNU/Linux 11 (bullseye)
Release: 11
Codename: bullseye
ll@leno:~$

Any ideas?

Thanks,
LeRoy


--
There is something glorious birthing within all of us.
I am the Love of God, no matter what.BSA
LeRoy Diener
213-LEROYIZ
213-537-6949
https://leroydiener.com

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