I have a similar problem as the asker in this question on Apple's forum.
After I updated from Sierra to High Sierra (v10.13.3) on a MBP retina 13" mid 2014, I noticed the character picker (special character pop-up) that appear when long pressing / holding a character, e.g. to type ë or ñ, does not appear anymore. However, when such a key is held, the character is repeatedly entered. This is not what I want, because I'm used to the character picker. I don't want to use ⌥ key with u for ¨, i for ^, e for ´ etc., which I fail to remember and is slower than simply holding to reveal any character variations. That is much more efficient in my opinions and therefore it is better UX.
I've tried the linked question's terminal command without success:
defaults write -g ApplePressAndHoldEnabled -bool true
Also, there's a comment in the discussion suggesting rebooting in safe mode and changing some keyboard settings might do the trick. I cannot boot into safe mode, possibly (?) due to administrative restrictions (company laptop).
So, how do I do it? Is the character picker a feature at all, or am I actually experiencing a bug?