When you create a Job it requests for credentials, username and password, make sure the username exists and the password is correct. Make sure the credential with the appropriate rights exist on the target database you are trying to execute the job. See this documentation about credentials for executing jobs.
You can update the Job to use a different credential, using PowerShell's Set-AzureSqlJob or you can update the credential if needed using Set-AzureSqlJobCredential. For more information how to use PowerShell with Elastic Jobs please visit this documentation.