Good day @AbhiMascot,
Thank you for posting to the PayPal community.
I would suggest to please contact your website developer or the third party eCommerce support to cross check your using same SANDBOX REST API Credentials(Client ID & Secret) while performing the below API calls.
https://developer.paypal.com/api/rest/authentication/
https://developer.paypal.com/api/rest/postman/
https://developer.paypal.com/docs/api/tracking/v1/
https://developer.paypal.com/docs/api/tracking/v1/#errors
Also, If a third party eCommerce partner is performing API operation on your behalf, you(i.e merchant) should be on-boarded and necessary API permissions should be granted to the Partner(API Caller).
https://developer.paypal.com/docs/multiparty/
https://developer.paypal.com/docs/multiparty/seller-onboarding/
Merchants can manually grant permission to a third party eCommerce to access specific information in your PayPal account and perform PayPal API operations on your behalf.
Login PayPal.com >> Settings >> Account Settings >> Website payments >> API Access >> Pre-built payment solution >> Grant API permission >> contact your "Third party software eCommerce" for Third Party Permission Username >> enter the username.
Direct link - https://www.paypal.com/cgi-bin/webscr?cmd=_profile-api-grant-authorization
https://developer.paypal.com/api/nvp-soap/paypal-payments-standard/admin/third-party/
If your still facing the issue, please create a MTS ticket via - https://www.paypal-support.com/s/?language=en_US with detailed error information.
Sincerely,
Kavya
PayPal MTS
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