synaptic package manager error: dpkg was interrupted, you must manually run 'sudo dpkg --configure -a' to correct the problem

Uh, what happens when you run exactly what it tells you to run?

jeff means put/copy paste sudo dpkg --configure -a in the terminal==>press enter==>password===>enter

You strictly need to open a Terminal from the menu Applications->Accessories->Terminal and type:

sudo dpkg --configure -a

give your user password when requested, you don't see nothing when you type it, then press enter.
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But to fully check and upgrade your system using terminal using command line,
open a Terminal from the menu Applications->Accessories->Terminal

Tip: right click with mouse on the terminal title caption and select the item "Always on Top" doing this you will force the terminal window to stay on top of the other windows and you will find very easy to copy single row from this web page into the terminal...
Something more about using the terminal https://help.ubuntu.com/community/UsingTheTerminal

Then type or better copy and paste a row a time then press enter:
(Tip: select the single row to copy then right click into the terminal and to quick paste click with middle button of mouse )

sudo dpkg --configure -a
sudo apt-get -f install
sudo apt-get --fix-missing install
sudo apt-get clean
sudo apt-get update
sudo apt-get dist-upgrade
sudo apt-get clean
sudo apt-get autoremove

give your user password when requested, you don't see nothing when you type it, then press enter.

Hope this helps