Hi,
Have you entered a sales tax group and item sales tax group when posting your journal? What transaction do you try to record, that is what account type do you use for the debit and credit? Depending on your setup and account type used it might be necessary to enter the sales tax code directly in the journal via the general tab of the journal. Would be great if you could provide some additional information if those hints don't help you out.
Regards,
Ludwig