Hello, I'm trying to make a REST API callout, with the use of named credentials. However when I try to make the callout, I get a 502 error. Am I missing anything basic (this is my first callout :)

public class EMedCalloutsExtension {
    
    public final Lead referral;
    public String requestLabel;
        public String result {get;set;}
        public List<Object> emedData {get;set;}
    

     Public EMedCalloutsExtension(ApexPages.StandardController stdController){
        this.referral = (Lead)stdController.getRecord();

         
  }
       public Object  makePostCallout() {
    Lead ref =[Select Id, firstname, lastname, gender__c, patient_dob__c, patient_ssn__c from Lead where Id = :ApexPages.currentPage().getParameters().get('id')]; 
           system.debug('***NAME OF REFERRAL***: '+ ref.firstname + ' ' + ref.lastname);
    String ssnReplace; 
           if (ref.Patient_SSN__c != null){
            ssnReplace = ref.Patient_SSN__c.replace('-','');
           }
    String genderLetter;
           if (ref.Gender__c == 'Male'){
               genderLetter = 'M';
           } else if (ref.Gender__c == 'Female'){
               genderLetter = 'F';
           }
           
           String first;
           String second;
           String third;
            String d = String.valueOf(ref.Patient_DOB__c);
           system.debug('***dob string raw: ' + d);
                first = d.substring(0,4);
                second = d.substring(5,7);
                third = d.substring(8,10);
           String dob = first + second + third;
           system.debug('***dob formatted: ' + dob);
           
           Map<string, string> patientInfoMap = new Map<string, string>();
                          patientInfoMap.put('social_security_number', ssnReplace);
                          patientInfoMap.put('birthdate', dob);
                          patientInfoMap.put('gender', genderLetter);
                          patientInfoMap.put('last_name', ref.lastname);
                          patientInfoMap.put('first_name', ref.firstname);
           List<Object> patientInfoList = new List<Object>();
           patientInfoList.add(patientInfoMap);
           
           JSONGenerator gen = JSON.createGenerator(true);   
            gen.writeStartObject();     
             gen.writeStringField('processing_mode', 'Realtime');
             gen.writeStringField('request_type', '270');
             gen.writeStringField('information_source_id', '1');
             gen.writeStringField('submitter_system_id', '2');
             gen.writeStringField('submitter_system_type', 'api');
             gen.writeStringField('submitter_request_id', 'R1');
                 gen.writeStringField('submitter_user_id', '3');
            gen.writeObjectField('subscribers', patientInfoList);
              gen.writeEndObject();   
         String jsonS = gen.getAsString();
        System.debug('***BODY:        '+jsonS);
          
       // Continuation con = new Continuation(40);
        //con.continuationMethod='processResponse';

       HttpRequest req = new HttpRequest();

req.setEndpoint('callout:Emed/');
   
        req.setHeader('ContentType', 'application/json'); 
  req.setHeader('Accept', 'application/json');         
System.debug('***HEADER:      ' + req.getHeader('Authorization'));
        req.setMethod('POST');
        req.setBody(jsonS);
       Http http = new Http();  
     HTTPResponse res = http.send(req);
                system.debug('Request: ' + req);

//print the header
    //get the list of header names (keys)
string[] headerkeys2 = res.getHeaderKeys();
//create an object to store your header key-value pairs
Map<string, string> headers2 = new map<string, string>();
//iterate through they keys, and populate your map
for(string s : headerkeys2){
   headers2.put(s,res.getHeader(s));
   system.debug('header of response: ' + s + ' value: ' + res.getHeader(s));
}

           
  String result = null;
    Integer statusCode = res.getStatusCode();
          system.debug('***statusCode is***: ' + statusCode);
   if(statusCode == 200){
             system.debug('***API invoked successfully***');
    result = res.getBody();
          system.debug('***RESULT***: ' + result);
      }   
           return result;   
        
    }
                                
 public Object processResponse() {  
        HttpResponse response = Continuation.getResponse(this.requestLabel);
 Integer statusCode = response.getStatusCode();
    system.debug('***statusCode is ***:' + statusCode);
        // Set the result variable that is displayed on the Visualforce page
        if (statusCode == 200) {
             system.debug('statusCode is ' + statusCode);
            this.result = response.getBody();
            Map<String, Object> results = (Map<String, Object>) JSON.deserializeUntyped(response.getBody());
            emedData = (List<Object>) results.get('emedData');
        }
        return null;
     
    }

}