@LakshmiKorapati
You mention that it works as a GET but not a POST, so you must be passing the authorization token somehow. The instructions appear on the top page for the API: Welcome to the Canvas LMS API Documentation. Typically, this is done with an Authorization: Bearer <token> header. For a GET, it can be included in the URL although that is discouraged.
OAuth2 is a way of getting a token, but you still need to include it with the request with the Authorization header. Tokens obtained through OAuth2 expire after an hour and it's up to the application to refresh the token to generate a new one.
Multiple grading periods are often used K-12 (in the U.S.) education. I'm in higher education, so I'm not all that familiar with them, but I think I read of people having issues changing grades outside of the grading period. If you have no idea what multiple grading periods are, then it probably doesn't affect you.
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