A couple of issues. I wanted to install a package (synaptic) but was unable to do this until I first did an 'apt update' and 'apt upgrade'. Then I was able to use 'apt install synaptic' and most other packages, but I next wanted to install the package libgtk-3-dev but there are some unmet dependencies.
The following package has unmet dependencies: libgtk-3-dev
Depends: libatk-bridge2.0-dev
Depends: libatk1.0-dev
Depends: libcairo2-dev
Depends: libgdk-pixbuf2.0-dev
Depends: libglib2.0-dev
Depends: libpango1.0-dev.
