Hello Khandakar Rashed Hassan,
Thank you for reaching out to Microsoft Support!
According to your question, we have conducted a test, the test results are as follows, the operation is successful.
For your problem, please do the following check:
If not, please share the details of your error request, and we will further help you analyze and solve the problem.
Reference document:
https://learn.microsoft.com/en-us/graph/api/planner-post-plans?view=graph-rest-1.0&tabs=http
Hope this helps.
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