The CreateRequest is used for a Create message, but you're trying to change the RequestName. To call an action, you should use a generic OrganizationRequest, not a CreateRequest
If you have an action called dev_CreateCustomer, then code within the action will need to set the SuppressDuplicateDetection parameter in a CreateRequest. You could control this by adding an input parameter called SuppressDuplicateDetection to the dev_CreateCustomer
action, but based on the error you haven't done this
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