Invalid grant_type and invalid credentials in OAuth 2.0 for Server to Server Applications for Google Drive

Question

After performing all the steps mentioned in: https://developers.google.com/identity/protocols/OAuth2ServiceAccount

and making the access token request using postman: https://www.googleapis.com/oauth2/v4/token

I received the following error:

"error": "invalid_grant",
"error_description": "Invalid JWT Signature."

Also I tried Service account authorization without OAuth, and received the error:

{"error": {

        "errors": [
            {
                "domain": "global",
                "reason": "authError",
                "message": "Invalid Credentials",
                "locationType": "header",
                "location": "Authorization"
            }
        ],
        "code": 401,
        "message": "Invalid Credentials"
    }
}

What could be the issue and how do I solve it?