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If you are using Partner's API, you need to make sure that your app has been granted permissions to process partner payments. If your app is not a Partner App, you cannot onboard sellers.
Here is the information on how to create a partner app:
After you have created an app with Partner Permissions, you can onboard sellers in Sandbox.
Here is a listing of sample code:
Here is PayPal's Demo Site with SDKS and demo working API (If you click API History as you go through the demo you can see the code):
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