The error here is mainly due to the request body having an invalid format of the DateTime properties.
Please specify the startDateTime and endDateTime as below.
{
"startDateTime":"2021-01-07T14:30:00.0000000-07:00",
"endDateTime":"2021-01-07T15:30:00.0000000-07:00",
"subject":"Test Graph"
}
It worked for me as you can see the below image.
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