If I am getting warning code 300 invalid customer id during ID sync, what could be the possible reasons? I have checked the documentation @ https://marketing.adobe.com/resources/help/en_US/aam/index.html?f=dcs_error_codes and what does it meant by "missing values for data source"?
This could be an issue because of something has been missed ID sync implementation.
ID sync stitches your CRM ID with AAM UUID. You need to check if the ID sync call is there soon after you log in ?
Are you using dpid and dpuuid (declared IDs) or d_cid_ic or d_cid in ID sync calls ?
How have you implemented ID sync, have you written a code to pick the variable for CRM ID value or are you using DTM ?
There is an article on implementing ID sync using DTM : Real time ID Sync in AAM using DTM, this will also give you an idea on debugging ID sync issues.