I had the same issue:
Could not get lock /var/lib/dpkg/lock - open (11: Resource temporarily unavailable)
Unable to lock the administration directory (/var/lib/dpkg/), is another process using it?
I tried searching for processes that were using it as advised in an earlier post by doing:
ps aux | grep apt
ps aux | grep ftp
Neither of those revealed the culprit.
I then noticed that I had Synaptic Package Manager open. Once I closed that and attempted sudo apt-get update it completed successfully.
Hopefully this helps!