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sudo apt-get update is giving me this error at the end. I'm using Y PPA Manager and it seems that now all PPAs are OK, but the problem (I think) is with the official Ubuntu repos:

W: GPG error: http://repository.spotify.com stable InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 13B00F1FD2C19886

W: GPG error: http://archive.ubuntu.com wily InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

W: GPG error: http://archive.canonical.com wily InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

W: GPG error: http://archive.ubuntu.com wily-updates InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

W: GPG error: http://archive.ubuntu.com wily-backports InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

W: GPG error: https://download.01.org vivid InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY A902DDA375E52366

W: GPG error: http://archive.ubuntu.com wily-security InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

W: GPG error: http://archive.ubuntu.com wily-proposed InRelease: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5 NO_PUBKEY 3B4FE6ACC0B21F32

Using Ubuntu 15.10

I've also tried this without success:

jose@jose-Lenovo-G400s:~$ sudo gpg --keyserver hkp://keyserver.ubuntu.com:80 --recv 40976EAF437D05B5

gpg: requesting key 437D05B5 from hkp server keyserver.ubuntu.com

gpg: key 437D05B5: "Ubuntu Archive Automatic Signing Key <ftpmaster@ubuntu.com>" not changed

gpg: Total number processed: 1

gpg:              unchanged: 1

jose@jose-Lenovo-G400s:~$ sudo gpg --export --armor 40976EAF437D05B5 | sudo apt-key add -

sudo: ignoring time stamp from the future

[sudo] password for jose: 

gpg: keyblock resource `/etc/apt/trusted.gpg.d/webupd8team_ubuntu_y-ppa-manager.gpg': resource limit

gpg: keyblock resource `/etc/apt/trusted.gpg.d/whatsapp-purple_ubuntu_ppa.gpg': resource limit

gpg: keyblock resource `/etc/apt/trusted.gpg.d/wine_ubuntu_wine-builds.gpg': resource limit

jose@jose-Lenovo-G400s:~$

I don't know what I'm doing wrong. Thanks.

User mchid answered a similar question stating that:

apt can only handle 40 keys in /etc/apt/trusted.gpg.d . 41 keys and you will get the GPG error "no public key found" even if you go through all the steps to add the missing key(s).

Check to see if there are any unused keys in this file from ppa(s) you no longer use. If all are in use, consider removing some ppa(s) along with the corresponding keyfiles in /etc/apt/trusted.gpg.d

So I went to /etc/apt/trusted.gpg.d and deleted 2 GPG keys (now with 39 left in /etc/apt/trusted.gpg.d). sudo apt-get update worked OK and now I can upgrade any package.