In the above image, you can't change the width as your selection also includes one or more MText entities with Columns - as Tom aptly described Columns must be set to None, then you can set for 0 width.
This may help to expedite the setting of Columns to None:
[Edit] - As well as set Width to 0.
Code:
(vl-load-com)
(defun c:MtextColumnsOffZeroWidth (/ *error* ss acDoc i e)
(defun *error* (msg)
(if s
(vla-delete s)
)
(if acDoc
(vla-endundomark acDoc)
)
(cond ((not msg)) ; Normal exit
((member msg '("Function cancelled" "quit / exit abort"))) ; <esc> or (quit)
((princ (strcat "\n** Error: " msg " ** "))) ; Fatal error, display it
)
(princ)
)
(if (setq ss (ssget "_:L" '((0 . "MTEXT"))))
(progn
(vla-startundomark
(setq acDoc (vla-get-activedocument (vlax-get-acad-object)))
)
(repeat (setq i (sslength ss))
(setq e (entget (ssname ss (setq i (1- i))))
e (append e '((75 . 0)))
e (subst '(41 . 0.0) (assoc 41 e) e)
)
(entmod e)
)
)
)
(*error* nil)
)
(princ)
Cheers