online order cancelled without responding to my emails

on the 26/11/2018 just place and order online and when i wanted to finalize the order i was getting error massage saying that the system cant authorize and i even send a email but they never responded now the order has been cancelled and i did call them to ask why they are not responding to my email and now the order has been cancelled and they told me i should have chosen 12 month instead of 24 month but i did send the email on time and they told me that they cant help me because they dont have stock but i went online there is stock in everything i wanted ,, i need those items . what is the use of having a account with you while you cant buy when items are on sale .