Make sure no other applications are using APT (synaptic, mintUpdate refreshing itself, mintInstall installing something..etc). Eventually, remove the file /var/lib/dpkg/lock yourself.
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sudo rm -rf /var/lib/dpkg/lock
By the way, you can type "apt upgrade" instead of "sudo apt-get upgrade" and you should really use mintUpdate instead unless you're sure you want to get all available upgrades.