Paypal Express is working but Payoal Direct raises an error

I am using Paypal module in my application. Paypal Express is working fine with real account but Paypal Direct raises an error with the same credentials!

Here is error details:
LongMessage: Security header is not valid
ShortMessage: Security error
ErrorCode: 10002

Would you please help me where is the problem?
Thanks in advance

make sure you are using the correct credentials for your account and make sure the 'use sandbox' is unticked if you are using a live account