How to change shortcut for switching languages to ctrl+shift in Ubuntu 20.04 KDE

I'm new to Linux, I want to use ctrl + switch to change languages, but it's not allowed from those settings. I use KDE.

This is where I tried changing the settings:

The screenshot from your question shows IBus Preferences configuration window.

While really you need KDE Keyboard Configuration. Access it as follows:

1. Open applications menu/dash

2. Find Keyboard Keyboard Hardware and Layout or launch using kcmshell5 kcm_keyboard

3. Go to Layouts tab, add necessary layouts

4. Go to Advanced tab and set Switching to another layout to Ctrl+Shift

   

5. Click OK

6. Enjoy.

Note: you may have problems with Ctrl+Shift+... shortcuts, this problem is fixable by using packages from my PPA.