Cannot use Dictionary
This is a bug in the macOS 10.12 Sierra GM (Golden Master).

Apple has released a few more builds since its keynote on September 7. The first build was 16A320.

To update to the current build there are two options:


Wait until macOS Sierra's release date (tomorrow, September 20)
Download the latest macOS Sierra installer from the developer center.


What is your current macOS Sierra build number?