apt-get update error unable to install mongo DB

it seems bad luck also likes me.! i’m trying to install mongo DB but facing issue while updating apt-get repository as below

Reading package lists… Done
E: The repository ‘http://repo.mongodb.org/apt/ubuntu bionic/mongodb-org/3.0 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. W: GPG error: http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY BC711F9BA15703C6 E: The repository ‘http://repo.mongodb.org/apt/ubuntu xenial/mongodb-org/3.4 Release’ 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://cz.archive.ubuntu.com/ubuntu precise Release: The following signatures couldn’t be verified because the public key is not available: NO_PUBKEY 40976EAF437D05B5

i suspect that something messed up with /etc/apt/sources.list.d Can anyone help me how could i resolve this and install mongo DB.

Many Thanks, Surya Kiran.