We have already resolved a similar query in this thread -
Let me know if it doesn’t resolve, we shall look into it then.
Please help me , it is not resolve yet.
I’d suggest you to recheck the Sandbox URL. These credentials are case sensitive. So make sure everything is accurate. if you still face the issue, create a new config file.
If it still doesn’t resolve, create a new mailgun account and redo the steps.
