/opt/librenms/poller.php 50 2018-09-20 17:51:03 - 1 devices polled

how to fix this error?

Where do you see the error?

tail -f /opt/librenms/log/librenms.log

No, I mean where do you see the error in:

/opt/librenms/poller.php 50 2018-09-20 17:51:03 - 1 devices polled

Thats not an error.