apt – Ubuntu 21.04 – Install a package (caffe-cuda) from Ubuntu 18.04

I have to install Caffe for a project in my university. I am new to Ubuntu and I am using it since I wasn’t able to build Caffe on Windows at all.

I have tried following the original tutorial. If I run sudo apt install caffe-cuda, I get the error Unable to locate package caffe-cuda. I understand, based on this, that this package is only available in 18.04 repositories. If I understand correctly, there is an easy way to get these packages, but unfortunately, after I run sudo apt update, a bunch of errors pop up, like

E: The repository 'http://archive.ubuntu.com/ubuntu bionic-security 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.
W: GPG error: http://archive.ubuntu.com/ubuntu bionic-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://archive.ubuntu.com/ubuntu bionic-updates 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.
W: GPG error: http://archive.ubuntu.com/ubuntu bionic InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 3B4FE6ACC0B21F32
E: The repository 'http://archive.ubuntu.com/ubuntu bionic 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.
E: The repository 'http://ppa.launchpad.net/danielrichter2007/grub-customizer/ubuntu hirsute Release' does not have a Release file.
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.

And if I run apt-cache search caffe, caffe-cuda is not there.

Right now, the only solution that comes to my mind is manually installing each package with its dependencies, but since every dependency has a dependency and so on, I was hoping there was an easier way.

I ask to you: am I missing something? What am I doing wrong? Why can’t I get 18.04 packages cleanly?