I have a csv file with Account records, that I'm trying to upsert with command line dataloader.
Specifying external Id field (in my case it's Load_Id__c)
my mapping file doesn't have any other relationship fields.
But I keep getting the MISSING_ARGUMENT error
Error message:Load_Id__c not specified
Is the field RMAcctNbr_c defined as an external ID in your Salesforce org?
Under Setup | Customise| Accounts | Fields then edit the field RMAcctNbr__c.
Under General Options there is a checkbox "External ID" which you must check if the field is an external ID. This will also index it, which can improve performance. If memory serves, you can have at most 3 external IDs per object.
Edit : Also as Mikey has pointed out all custom field names end double underscore c __c so check how you have entered the field names.
A detail that I almost don't see people talking about is that when entering the password, you must enter "password+security token " that token that you get inside salesforce
Best Answer
In order to solve such error you should check very carefully the case of field names in your mapping and configuration files! The case of destination field names must be the same as in the destination org.
In my case I had Load_ID__c instead of Load_Id__c (as in the destination org) in the configuration file.