The public key is not available: NO_PUBKEY 68818C72E52529D4

After adding a mongodb you may experience an error about the GPG PUB KEY like teh following.

W: GPG error: http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/testing Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 68818C72E52529D4
E: The repository ‘http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/testing Release’ is not signed.

You can fix this issue by adding the PUB KEY for MongoDB’s repository.

apt-key adv --keyserver keyserver.ubuntu.com --recv-keys 68818C72E52529D4

Try update again

apt update