Broken Promises!!!

I'm a very unhappy customer. After putting in a laybye at Game Greenstone on 29th February 2020 and making final payment at the end of May, on item number 768694 I have been given the run around by the store, telling me that they do not have the item in stock. I was told that when putting in a laybye, the store would put aside the item as it was "in the process of being bought". I am at my wits end with the store and I don't know what else to do regarding this!