I had this problem when I had a typo in my .tmux.conf. It needed to say
set-window-option -g mode-keys vi but it said
set-window-option -g mode-keys vim. Of course, I did not have any keys bound for "copy-mode-vim".
If you recently switched versions of tmux from 1.x to 2.x, the tmux.conf syntax may have changed for you as well. You'd see errors on tmux startup if that is the case.
As a reference, if you're interested, I have the following working keybindings show up when using :list-keys
Puts you into copy mode with Ctrl+b PgUp
bind-key -T prefix PPage copy-mode -u
Uses PgUp and PgDn when already in copy-mode:
bind-key -T copy-mode NPage send-keys -X page-down
bind-key -T copy-mode PPage send-keys -X page-up
Used PgUp and PgDn when already in copy-mode-vi
bind-key -T copy-mode-vi NPage send-keys -X page-down
bind-key -T copy-mode-vi PPage send-keys -X page-up
Posting the output of
C-b :list-keys and your the contents of your
~/.tmux.conf would help.