I followed Jason's direction.
I'm pasting below from my CLI what came from running update and upgrade. It looks like Jason was right. There are some interesting results related to kdenlive, etc. I'm not sure what I should do now.
$ sudo apt update
[sudo] password for ll:
Get:1
http://security.debian.org/debian-security stretch/updates InRelease [94.3 kB]
Get:2
http://security.debian.org stretch/updates InRelease [94.3 kB]
Get:3
http://ppa.launchpad.net/kdenlive/kdenlive-stable/ubuntu focal InRelease [15.9 kB]
Err:3
http://ppa.launchpad.net/kdenlive/kdenlive-stable/ubuntu focal InRelease
The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2763B0EE7709FE97
Ign:4
http://debian.osuosl.org/debian stretch InRelease
Get:5
http://debian.osuosl.org/debian stretch-updates InRelease [91.0 kB]
Hit:6
http://debian.osuosl.org/debian stretch Release
Reading package lists... Done
E: The method driver /usr/lib/apt/methods/https could not be found.
N: Is the package apt-transport-https installed?
W: GPG error:
http://ppa.launchpad.net/kdenlive/kdenlive-stable/ubuntu focal InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 2763B0EE7709FE97
E: The repository '
http://ppa.launchpad.net/kdenlive/kdenlive-stable/ubuntu focal InRelease' is not signed.
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.
ll@leno:~$ sudo apt upgrade
Reading package lists... Done
Building dependency tree
Reading state information... Done
Calculating upgrade... Done
The following packages were automatically installed and are no longer required:
icedtea-netx icedtea-netx-common linux-image-4.9.0-7-amd64
linux-image-4.9.0-8-amd64
Use 'sudo apt autoremove' to remove them.
The following packages have been kept back:
kdenlive-data libmlt++3
The following packages will be upgraded:
libmlt-data
1 upgraded, 0 newly installed, 0 to remove and 2 not upgraded.
Need to get 85.2 kB of archives.
After this operation, 33.5 MB disk space will be freed.
Do you want to continue? [Y/n] y
WARNING: The following packages cannot be authenticated!
libmlt-data
Install these packages without verification? [y/N] n
E: Some packages could not be authenticated