Did you put the accent above the last letter? "poslední" and not "posledni"?
Is n a soft consonant? I thought they were c, j, and all the consonants with accent marks over them. In other words, why does this end with i instead of e?
N followed by I or Í is actually a Ň, see the Tips and notes about Czech orthography and pronunciation or any other similar resource. https://www.duolingo.com/skill/cs/Phrases/tips-and-notes (almost at the end)