You can avoid a penalty from the tax office if the tax authorities haven't done anything yet. The Ministry of Finance explains

The Ministry of Finance clarified that the lack of a tax declaration in the tax authority's system does not constitute documented evidence of a tax offense, allowing for the possibility of a voluntary disclosure (czynny żal). This disclosure informs the tax authorities of a missed obligation, such as failing to submit an annual tax return. However, if the tax authorities have already initiated checks or contacted the taxpayer, the voluntary disclosure becomes ineffective. Taxpayers must submit their declarations promptly and address any owed amounts to avoid penalties. Corrections to submitted returns can be made without a voluntary disclosure if done before any legal proceedings begin.