Which version exactly of Sierra? See Apple Menu > About This Mac… .
Is Generate > Pluck… and Effect > Adjustable Fade… also missing? If so, ensure at Audacity > Preferences…, Effects section that that the “Nyquist” box is ticked (checked).
Ensure you installed Audacity correctly and are not running it from the DMG: Installation: Do NOT run Audacity from the DMG!!!!
If that does not help, it is probably Gatekeeper interference, assuming you are on macOS 10.12.1 or later. Open /Applications/Utilities/Terminal.app. Copy the below text then paste it into the terminal:
sudo xattr -r -d com.apple.quarantine /Applications/Audacity/
Hit ENTER to send the command. Type the admin password when prompted. You won’t see the password while typing it, but hit ENTER when you have finished typing it.
Then reboot.
If Audacity is installed somewhere else than /Applications/Audacity, change the path to reflect where the Audacity folder is.
As it says in the 2.1.2 Release Notes:
(OS X) Audio files cannot be dragged into the Audacity window. You can drag files to the Audacity icon in /Applications or in the Dock, or import the file from within Audacity
This will be fixed in Audacity 2.1.3 when released.
Gale