Not familiar with FvwmCommand and I am also not sure if it does what I have read. FvwmConsole commands to run in a terminal? I have a bash script that requires “Restart” Fvwm.
FvwmCommand Restart gives the same error posted by others.
Cannot find FvwmMFL socket None.
Please start FvwmMFL module from your fvwm configuration.
FvwmCommand manpage tells to run:
AddToFunc StartFunction "I" Module FvwmCommandS , still gives same error.
(Fvwm3 version 1.0.4-122)
If you just want to restart FVWM, killall -SIGUSR1 fvwm3 may be easier.
I start FvwmConsole from a menu, but with basically the same command, and it works for me.
DestroyMenu FvwmMenu
AddToMenu FvwmMenu
+ "FVWM Menu" Title
+ "FVWM &Console%mini.term.xpm%" Module FvwmConsole -bg blue -fg white
That menu has worked for me in fvwm2 2.6.8 (and quite a few versions prior), and now in fvwm3 1.0.5 (1.0.4-135-gc8763635).
Additionally, issuing the simple command Module FvwmConsole in the FvwmConsole window, opens a second console window.
Sorry for the late reply… thanks, it works well. Didn’t know about “SIGUSR1”, still learning. 
This is fixed in latest Fvwm3 1.0.9 GitHub update. To run Fvwm commands in the terminal. Or even better, to mix commands in Bash scripts.
#!/bin/sh
echo "This file restarts fvwm"
FvwmCommand Read ~/.fvwm/test.sys
It is disingenuous to state that this was fixed in 1.0.9, for two reasons. First it was never broken (you have to load the module FvwmMFL for FvwmCommand or FvwmPrompt to work), and second the only change was to load FvwmMFL from the default-config, which will have no effect on users who don’t use the default-config.
Just load Module FvwmMFL from your StartFunction if you want to use FvwmCommand, which is exactly what the error you get says if it isn’t loaded.