There are a few reasons that sync errors happen in Monitor, the two most common are (1) loss of connection while syncing or (2) missing or invalid data. 
To try and fix a sync error yourself, check first that you have a strong and stable internet connection, including checking that your device is not on flight mode, and then try to sync again. Make sure not to navigate away from Monitor while the device is syncing. Syncing can take a few minutes, longer if you are syncing many large datasets, so give it some time. If your sync is still unsuccessful, or there is an 'invalid data' sync error, then you will need to submit a ticket to our helpdesk team.