Exception:

{
    "error": "invalid_request",
    "error_description": "must use HTTP POST"
}

Resolution:

We get this exception when the URL is incorrect with lightning domain URL instead of the Salesforce My Domain URL in the HTTP Requests. Use you Salesforce My Domain URL instead of the Lightning Experience URL to resolve or fix this issue.

Correct URL: https://{Your_Salesforce_Domain}.my.salesforce.com

Incorrect URL: https://{Your_Salesforce_Domain}.lightning.force.com