Neb. Rev. Stat. § 77-2796

If the Tax Commissioner disallows a claim for refund, he or she shall notify the taxpayer accordingly. The action of the Tax Commissioner denying a claim for refund is final upon the expiration of thirty days after the date when he or she mails notice of his or her action to the taxpayer unless within this period the taxpayer seeks review of the Tax Commissioner's determination as hereinafter provided.