Am I sinful for previously denying, unknowingly, the Hadith of the Card and the Scrolls? What is the ruling on one who denies authentic hadiths? And what should I do if I am sinful?

You erred by rushing to issue a fatwa and denying something you did not know to be true before verifying it. You must repent. If the meaning is unclear to you, say, "Perhaps the hadith is weak," rather than denying it without knowledge. You did not deny an authentic hadith, but you spoke without knowledge, so repent to Allah and do not return to such behavior. Denying authentic hadiths is a greater crime than speaking without knowledge.