How to assign ctrl_shift to switch layout in kde kubuntu 20.10?

Question

Kubuntu

How to assign ctrl_shift to switch layout in kde kubuntu 20.10?
I go to the system settings / keyboard shortcut / switch layouts / select the next layout
I try to change by pressing ctrl_chift, it doesn’t change
the feeling that these keys are already assigned
somewhere, I put ctrl_space, everything is ok
, I also tried it through the system tray, configure the keyboard layout widget,
the same story,
tell me how appoint