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From the error message you received, it looks like the OwnerId column is not mapped correctly.
Please see the Soap API developer’s guide to check what is the relevant content of the OwnerId value:
This means that you should pass User.Id instead of a User.Name to this column.
You can use a query below to receive a required value that can be specified as a Constant value in mapping or in the CSV file:
SELECT Id, Username
The second option is to set up lookup mapping for this column. For example, if there is a username field in the CSV file (but there is no Id), then you can set Target lookup mapping in this case.
If you have a Name column, storing the user name for each record, in the file, here is an example of the mapping that you can use.
The mapping searches a record in the User table with Username = Name and takes its ID, which is then substituted into the OwnerId field.
Please check the guide below to learn more about Lookup Mapping:
Please let us know if we may help you more.