The following exception occurs when SendEmail from Case object is used and contextId is not passed. To resolve the issue, please add contextId to the POST body.

message: performQuickAction for Case.SendEmail requires a parent object
errorCode: MISSING_ARGUMENT

Sample Code with contextId:
{    
    “contextId”: “5008c00001Ejue0AAB”
    “record”: {
        “HtmlBody”: “<div+style=”font-family:+Arial;+font-size:+14px;”>Test</div>”,
        “Subject”: “InfallibleTechie”,
        “ToIds”: [“0038c00002bkSPZAA2”]
    }
}