[Python] Microsoft Graph API's "Access is denied. Check credentials and try again"

Hello everyone i'm new on Microsoft Azure platform. We are using mircosoft outlook graph api for send mails. The credentials and things are working fine. But when we are using api with /sendMail method it returns.
​

Access is denied. Check credentials and try again.

authority = f"https://login.microsoftonline.com/{tenant_id}"
app = msal.ConfidentialClientApplication(
client_id=CLIENT_ID,
client_credential=CLIENT_SECRET,
authority=authority)

scopes = ["https://graph.microsoft.com/.default"]
result = None
result = app.acquire_token_silent(scopes, account=None)

if not result:
result = app.acquire_token_for_client(scopes=scopes)

if "access_token" in result:
print("Access token created.",result["access_token"])

if "access_token" in result:
endpoint = f'https://graph.microsoft.com/v1.0/users/{userId}/sendMail'
toUserEmail = user_email
email_msg = {'Message': {'Subject': "Test Sending Email from Python",
'Body': {'ContentType': 'Text', 'Content': "This is a test email."},
'ToRecipients': [{'EmailAddress': {'Address': toUserEmail}}]
},
'SaveToSentItems': 'true'}

r = requests.post(endpoint,headers={'Authorization': 'Bearer ' + result['access_token']},json=email_msg)
if r.ok:
print('Sent email successfully')
else:
print(r.json())```

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