You'll have to contact Airbnb customer support to discuss the issue.
Ways to contact them can be found in the post below. I found it by typing "contact airbnb" in the "Search the community" box that appears on these forum pages where you posted your question.
@Branko7 na dnu maila kojim te obavještavaju o kuponu trebalo bi pisati koliko dugo vrijedi. Mi smo isto dobile kupon i piše da nam vrijedi godinu dana, čudilo bi me da tebi vrijedi kraće.