And when I assign other command to left shift it display error, and when I press the sign it redirect me to the keyboard shortcut, which does not display any conflict...
I just want to use my right shift ToT
I just had the same issue! I am using an external keyboard, btw, but I believe this fix should be the same.
First I just went ahead and restarted, then after my right shift was still acting wonky, I went into the Keyboard Shortcuts preferences, and changed the shortcut to numpad0. I saved it and did a restart again, then it was fixed.
I'm not sure if using the numpad altered the issue, or if all it took was multiple restarts, but before that, even if I disabled "Show Desktop" it would still be active as my Right Shift.
On a weird note, my show desktop used to be a 3 finger swipe down on the trackpad, but now that option isn't even available for me to select. Odd.