You should change this in add_wcs_to_conference.sh script too:
curl -s -X POST -d "{ \"callId\": \"5620-$CONF_NUMBER\", \"dtmf\": \"1\", \"type\": \"INFO\" }" "$URL/send_dtmf"
We've done it, and the error gone, Asterisk displays in logs
[2021-09-29 23:13:10] VERBOSE[19955][C-00000301]...