I think this is your problem:
"Position/X
I'd be tempted to use entmod instead of setpropertyvalue - see the first text to change as an example, commented out your original lines
(defun c:mycoverpage (/ ss ss1 ss2 e e1 e2 )
(if (setq ss (ssget "_:S" '((0 . "MTEXT"))))
(Progn
(setq ed (entget (ssname ss 0 )) )
(setq ed (subst (cons 40 15) (assoc 40 ed) ed ))
(setq ed (subst (cons 10 (list -2959.684 730.225 0)) (assoc 10 ed) ed ))
(entmod ed)
; (setq e (ssname ss 0 ))
; (setpropertyvalue e "TextHeight" 15.00)
; (setpropertyvalue e "Position/X" -2959.684)
; (setpropertyvalue e "Position/Y" 730.225)
; (setpropertyvalue e "Position/Z" 0.00)
)
)
(if (setq ss1 (ssget "_:S" '((0 . "MTEXT"))))
(Progn
(setq e1 (ssname ss1 0 ))
(setpropertyvalue e1 "TextHeight" 14.00)
(setpropertyvalue e1 "Position/X" -2957.874)
(setpropertyvalue e1 "Position/Y" 689.120)
(setpropertyvalue e1 "Position/Z" 0.00)
(setpropertyvalue e1 "Contents" "\\pxqc;{\\C256;DETAILED DESIGN}")
)
)
(if (setq ss2 (ssget "_:S" '((0 . "MTEXT"))))
(Progn
(setq e (ssname ss2 0 ))
(setpropertyvalue e2 "TextHeight" 13.00)
(setpropertyvalue e2 "Position/X" -2674.324)
(setpropertyvalue e2 "Position/Y" 568.830)
(setpropertyvalue e2 "Position/Z" 0.00)
(setpropertyvalue e2 "Contents" "{\\fArial|b0|i0|c0|p34;\\C256;MARCH 2023}")
)
)
(princ))