Are you trying to do this in GNOME or in Xfce or somewhere else ?

Gnome

I've the same issue. Im using Gnome and i dont have the lock icon too. I only have settings and power button

Have you checked your shortcut keys?

Yes.. i tried Super(window) + L key and changed to Ctrl + L with the same result

Another note: running gnome-screensaver-command --lock from terminal i got kali locked but Lock Button still missing in System Menu

Another note: running gnome-screensaver-command --lock from terminal i got kali locked (but only after i run gnome-screensaver &)

Lock Button still missing in System Menu

PS: I cant edit previous note

apt-get -f install --reinstall gnome-shell or apt-get -f install --reinstall gnome-shell-extension and https://extensions.gnome.org/extension/930/suspend-and-lock-button/ have you tried?

also have gnome-tweak installed?

I have gnome-tweaks installed and done this "apt-get -f install --reinstall gnome-shell*". when I now click Super+L or gnome-screensaver-command --lock my screen goes blank and when i move my mouse it shows the current screen. does not lock thus does not ask for password

locks for other accounts but not for root. a solution online said i should do this
https://forums.kali.org/showthread.php?1660-Root-screen-locking
how safe is it? any issues?
actually found the specified code in gnome-screensaver/src/setuid.c
if (getuid () == (uid_t) 0) {
reason = g_strdup ("running as root");
ret = FALSE;
goto out;
}
do i comment it out?

It's precisely to avoid that kind of issues that we're moving away from using the root user. See https://www.kali.org/news/kali-default-non-root-user/

Thus we're not going to fix this specific issue.