SendEmail Web API ERRORS: "No HTTP resource was found that matches the request URI" and "')' or ',' expected at position..."

I'm trying to create an Email record and then send it using JS. Here is a portion of my code:

    Xrm.WebApi.createRecord("email", data).then(

        function success(result) {
            var emailId = result.id;
            emailId = emailId.substring(1,emailId.length-1);

            window.parentFormContext.getAttribute("new_giresponseemail").setValue("/emails(" + emailId + ")");

            var url = 'https://ORGNAME.crm.dynamics.com/api/data/v9.1/emails(' + emailId + ')/Microsoft.Dynamics.CRM.SendEmail';

            var body = JSON.stringify({
                "IssueSend": "true"
            });

            var req = new XMLHttpRequest();
            req.open("POST", url, true);
            req.setRequestHeader('Content-Type', 'application/json');
            req.send(body);

            Xrm.Utility.showProgressIndicator("Please wait a moment...");
            req.onreadystatechange = function () {
                Xrm.Utility.closeProgressIndicator();    
                if (this.readyState === 4) { 
                    req.onreadystatechange = null;
                    if (this.status === 200) {
                        var result = "Response successfully sent!"; 
                        alert(result); 
                    }
                    else if(this.status === 400){
                        var result = "ERROR: Please contact your CRM administrator."; 
                        alert(result);
                    }
                }
            }  
        },

        function (error) {
            console.log(error.message);
        }
    )

This creates the email record correctly. However, when the XMLHTTPRequest is opened I'm getting a 400 response with the error message "')' or ',' expected at position 4 in (GUID)" for the API action. Sometimes the "position" number is different depending on the GUID that was generated for the new email record.

If I enter the API URL in the browser manually I get this error: "No HTTP resource was found that matches the request URI '">ORGNAME.crm.dynamics.com/.../Microsoft.Dynamics.CRM.SendEmail'."

I've scoured the internet trying to find a solution for this but no luck. Thanks for any help!