Both "mark as complete" and "mark as incomplete" are mapped to the key sequence ctrl+m, so when you press that shortcut a dialog says "The key sequence 'Ctrl+M' is ambiguous. Use 'Configure Shortcuts' from the 'Settings' menu to solve the ambiguity. No action will be triggered."
Hello! Sorry to be the bearer of bad news, but this project has been unmaintained for many years so I will be closing this bug.
Fixed in the KF5 port: https://cgit.kde.org/ktimetracker.git/commit/?id=7d2db34db3e5fac7fb882fd85d79a6f65ff4aa2a