elementary tweaks not installing

[manu sharma]

I am trying to install elementary tweaks on Acer Aspire V5-472.

After I use:

sudo apt-add-repository ppa:versable/elementary-update
sudo apt-get update

I get this error message:

W: GPG error: http://ppa.launchpad.net precise Release: The following signatures couldn't be verified because the public key is not available: NO_PUBKEY 4F191A5A8844C542
E: Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable)
E: Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?