Perhaps these lines from Debian 11's /etc/festival.scm are not added to your install and may help?
;; Debian-specific: Use aplay to play audio
(Parameter.set 'Audio_Command "aplay -q -c 1 -t raw -f s16 -r $SR $FILE")
(Parameter.set 'Audio_Method 'Audio_Command)
In any case, I do not see this error on Debian.I did however go through the motion of installing the nitech voices before reading that they only work with older versions of festival. Doh!