hi everyone,
I am trying to integrate SharePoint using apex
I generate an access token from the postman and copy the access token and then pass it to my apex method it works but when I generate into apex class and pass to method it is not working

public class Rg_SharepointIntegration {
    public static void getconnection(){
        String clientId = '*****************'; 
        String clientSecret = 's**************';
        String tenantId = 'c*****************72';
        Http h = new Http();
        HttpRequest req = new HttpRequest();
        req.setMethod('POST');
        req.setHeader('Content-Type', 'application/x-www-form-urlencoded');
        //https://login.microsoftonline.com/< tenant Id>/oauth2/token
  req.setEndpoint('https://login.microsoftonline.com/'+EncodingUtil.urlEncode(tenantId, 'UTF-8').replaceAll('\\+', '%20')+'/oauth2/token'); 
        String body = 'grant_type=client_credentials'+
                        '&scope=files.readwrite.all'+
            '&client_id=' + EncodingUtil.urlEncode(clientId, 'UTF-8') +
            '&client_secret='+ EncodingUtil.urlEncode(clientSecret, 'UTF-8') +
            '&resource=https://graph.microsoft.com';                              
        req.setBody(body);
        HttpResponse res = h.send(req);
        
        Map<String,String>connectionMap = new Map<String,String>();
        String accessTockn;
        if(res.getStatusCode() == 200){
            System.debug('Response Body: ' + res.getBody());
            connectionMap =  (Map<String, String>) Json.deserialize(res.getBody(),Map<String, String>.class);
            System.debug('connectionMap '+connectionMap);
            if(connectionMap.containsKey('access_token') && connectionMap.get('access_token') != null){
                accessTockn =connectionMap.get('access_token');
              getChildren(accessTockn);
            }
        }
    }
    public static void getChildren(String access_token){
        //String access_token= 'access_token';
        Http h = new Http();
        HttpRequest req = new HttpRequest();
        req.setMethod('GET');
        req.setHeader('Authorization', 'Bearer '+ access_token);
        req.setHeader('Content-Type', 'application/json');
        req.setHeader('Accept', 'application/json');
       //req.setEndpoint('https://graph.microsoft.com/v1.0/drives/');
       req.setEndpoint('https://graph.microsoft.com/v1.0/me/drive/root/children');
        HttpResponse res = h.send(req);
        System.debug('Response Body: ' + res.getBody());
        //Get your drive id from the response and store it somewhere.
    }    
}

above code returns an error response in getChildren(accesstoken) 
 "message": "Unable to retrieve user's mysite URL.",

but in the  same method getChildren('accesstoken_generated from Postman')  here I pass postman generated access token it works fine
don't know what I am missing in my apex request for an access tokenThanks,
Rahul