If you are experiencing issues with Mission Control on Mac version 10.15.13 after an update, there is a fix that involves using the Terminal app. Simply type or copy the following command: "defaults write com.apple.dock mcx-expose-disabled -bool FALSE", then type "killall Dock" to stop the Dock, which will automatically restart. After this, the Exposé activation should take effect.
Go to the Terminal app
Type or copy: defaults write com.apple.dock mcx-expose-disabled -bool FALSE
Then type: killall Dock to stop the Dock that will then automatically be restarted. Only then the Exposé activation will take effect.