Based on the error message, your distribution is not running xfwm4 by default. Rather it is running Mutter which is the Gnome window manager. So, not being able to open Window Manager and Window Manager Tweaks is likely that way by design. In order to open these, you would need to change your window manager to xfwm4. However, I would proceed with caution because this could break some functionality intended by the distribution developers.
If you decide to switch to xfwm4, make sure it is installed first. I cannot imagine it not being installed, but you can check with the command
which xfwm4. If installed, it'll return the path
If changing your window manager for some reason does not work and you do not have a window manager functioning after attempting to switch, open a terminal and run the command
xfwm4 --replace. (However, beforehand, make sure you have a keyboard shortcut set to open a terminal in Keyboard > Application Shortcuts.) Then open Settings Editor > xfce4-session > /sessions/Failsafe/Client0_Command. Click on the setting there and click the Reset button at the bottom.
Here is a post on changing the window manager in Xfce.
This command worked on my system for changing the window manager to xfwm4:
xfconf-query --channel xfce4-session --property /sessions/Failsafe/Client0_Command --type string --set 'xfwm4' --type string --set '--replace'