I got an error file that says "Cannot specify any additional fields when marrying or separating a Person-Account".

You are receiving the error message because you have previous business accounts in Salesforce for Shopify customers and now you have changed the mapping to Person Account.
 
As a result, Salesforce can’t update Business Accounts to Person Accounts through API.
 
You can solve this issue either by deleting those business accounts of Shopify Customers in Salesforce or changing Record-Type by clicking the button on those records.

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