A problem that I recently experienced is that Mission Control wasn’t working on macOS Sierra anymore. It didn’t respond to the touchpad gestures, the function key on the keyboard or the launching it from the applications menu.
One of the first things I always try when something isn’t working, is rebooting the system. But that didn’t help in this case. Also the settings in System Preferences looked all normal.
On Google I found the following, which worked for some people. So you can always try this. Launch a terminal session, which you can find in your applications and then type the following followed by pressing the return key:
This will restart the Dock and for some people this helps to get Mission Control working again. But in my case it unfortunately didn’t help.
After doing some more research I found the application OnyX. Which you can download for free. Install it by dragging it to your Applications folder. Launch the application. It will probably ask you if you will allow it to launch, because it has been downloaded from the internet. If is being blocked press OK and go to System Preferences -> Security and Privacy. At the bottom of the General tab it will say something about the blocked App OnyX, followed by a button with Open Anyway. Press this button and OnyX will be launched. It will ask you for your credentials, because it needs Administrator privileges to read and change system settings. So fill in your password and continue.
At start accept the license agreement. You’ll be asked to scan the startup disk. If you like you can allow this, but this can take some time. For fixing this issue it isn’t necessary, so I chose cancel. Go to the Parameters tab and chose Applications. It will shortly scan for the settings. If you had the same issue has I had, you’ll find the box for Enable Mission Control unchecked. Make sure you check it and at the popup press continue. In the screenshot an example (in Dutch). Your mission controle should now be working again.