Article 21 of Decree-Law No. 157 of 2024 grants taxpayers the right to amend a previously filed tax declaration. This action is permissible if the taxpayer discovers an omission, or a material or arithmetic mistake, in their original filing. The amended declaration must be submitted within five years from the statutory deadline of the original declaration. A key condition is that no formal tax assessment has yet been issued for the specific tax period in question. It is crucial to note that filing an amendment does not automatically grant immunity from applicable fines and penalties stipulated within the law.