"ORA-01555: snapshot too old" is an Oracle error. Many good sites, such as this one, explain what this error means in general.

More specifically when Luminate Online starts a sync it takes a snapshot of the database so that changes are made against a consistent set of data from that point in time. 

If the sync takes too long to run, the snapshot becomes invalid, it expires. This is usually the cause of this error on Luminate Online.

If this happens occasionally, restart the sync manually.

If this happens regularly, contact support to see if anything  can be done - your sync may need to be enhanced through a Data Integration project to keep this from happening.