Display accents on Touch Bar when holding a letter under Mojave
There is a possible reason why this may be:

You've installed the third-party product Webroot SecureAnywhere.


  Webroot SecureAnywhere is made by Webroot Inc. When this software is
  installed on your Mac using its default settings, you might notice
  these symptoms:
  
  Text that you dictate doesn't appear as expected. You don't see any
  accented characters when you hold down a letter key. You can't select
  items such as emoji in the Touch Bar of your 2016 MacBook Pro.
  
  Follow these steps to fix the issue:
  
  Click the Webroot SecureAnywhere icon in the menu bar. Choose Pause
  Secure Keyboard Entry.