Failed to fetch https://cloud.r-project.org/bin/linux/ubuntu/focal-cran40/InRelease The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 51716619E084DAB9 code example

Example: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY

sudo apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 6494C6D6997C215E