I tried it with a new account but showing the same error.
The findCompletedItems call is deprecated as of October 15, 2020, and access is restricted.
An alternative to the findCompletedItems call is the Marketplace Insights API, but please note that this API is restricted and requires eBay Business approval for access. Please contact the Developer Technical Support team from here: https://developer.ebay.com/my/support/tickets if you are interested in getting access to the Marketplace Insights API.
Please visit API Deprecation Status page for more details:
eBay Developer Support
This call has been deprecated.
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