<PC> has signed in message on screen, but if I try to connect to the remote PC message "A connection couldn't be established. Teamviewer is not running on remote PC appears.
Teamviewer 12 on both
On the remote PC, does TeamViewer say "Ready to Connect" on the main screen?
Check if you have "easy connect" enabled (I'm not sure about "easy connect" because I use a portugese version, but it's a acceptabel translation...)