I had the same issue, for me it was because I had not updated the location of the password file in the mqtt config, Check step 5 in the SD build. If you are installing manually I recommend reviewing the SD image build instructions. There are a few steps here that I think are missing on the other build instructions.
Turn off Mosquitto persistence and enable authentication:
sudo nano /etc/mosquitto/mosquitto.conf
Set persistence false and add the lines:
allow_anonymous false
password_file /etc/mosquitto/passwd
Create a password file for MQTT user emonpi with:
sudo mosquitto_passwd -c /etc/mosquitto/passwd emonpi
Enter password emonpimqtt2016 (default)
Test MQTT
Open another shell window to subscribe to a test topic:
mosquitto_sub -v -u ‘emonpi’ -P ‘emonpimqtt2016’ -t ‘test/topic’
Publish to the test topic :
mosquitto_pub -u ‘emonpi’ -P ‘emonpimqtt2016’ -t ‘test/topic’ -m ‘helloWorld’