How to solve the "no public key available..." error when using apt-get on Debian/Ubuntu

When running apt-get update to refresh the update source in Ubuntu and Debian, the problem of "There is no public key available for the following key IDs" is often encountered, which is manifested as the following error message:

 W:There is no public key available for the following key IDs: 3B4FE6ACC0B21F32, W:There is no public key available for the following key IDs: 3B4FE6ACC0B21F32,

The solution is as follows,

 sudo apt-key adv --recv-keys --keyserver keyserver.ubuntu.com 3B4FE6ACC0B21F32

Replace 3B4FE6ACC0B21F32 with the IDs you have problems with.

In addition, if you have the following problems, it is because you have added the natty-backports source, just delete it

 W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/main/source/Sources 404 Not Found [IP: 91.189.91.13 80], W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/restricted/source/Sources 404 Not Found [IP: 91.189.91.13 80], W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/universe/source/Sources 404 Not Found [IP: 91.189.91.13 80], W:Failed to fetch http://us.archive.ubuntu.com/ubuntu/dists/natty-backports/multiverse/source/Sources 404 Not Found [IP: 91.189.91.13 80],

Run the following command:

 sudo sed -i '/natty\-backports/d' /etc/apt/sources.list{,.d/*}

via: https://www.vmvps.com/apt-get-error-no-public-key-available-solution.html