cannot access long press and hold function for accents

[Kuba77]

I can't long press and hold a key to get the accents box any more. I'm learning german and getting umlauts this way was helpful. When I'm trying to open accents box, keyboard freezes (stops responding) in app I'm currently working. I mean, when I'm trying to long press s (to get ß) in Safari, keyboard freezes and I have to close Safari and open it again to unlock keyboard.

I've found this solution on the internet:
open up the terminal and type:
defaults write NSGlobalDomain ApplePressAndHoldEnabled -bool true
but it's not working in my case...

edit: I've created new test account and there's no problem with accents box on this account...

I'm using Macbook Air (early 2015), Mojave fully updated

 

[Kuba77]

Problem solved - I've used Onyx system check and it helped. I don't know what exactly was the problem.