401 errors are authentication errors and "Specified signature is expired" is an error that occurs when the generated HMAC Signature has expired.
Since HMAC signatures last max 5 minutes from creation, HMAC signatures must be newly generated for every Coupang API call.
[Guide to generate HMAC signature] provides examples to generate HMAC.
If there is no problem with the program, please check whether the expiration date (180 days) of the OpenAPI key has expired.