Basic troubleshooting questions, sorry if I’m being too basic…
Did it start up and work properly before setting up the username and password?
If you change “allow_anonymous” to true (in /etc/mosquitto/conf.d”) will it start?
My .conf file on the Mosquitto broker that I’m using a username and password on is shown below, if that helps.
pid_file /run/mosquitto/mosquitto.pid
persistence true
persistence_location /var/lib/mosquitto/
log_dest file /var/log/mosquitto/mosquitto.log
include_dir /etc/mosquitto/conf.d
allow_anonymous false
listener 1883
password_file /etc/mosquitto/pwdfile