not giving what they promised

I applied promo code lap6k for my order no 805513380 and they promised me a cash back of 6000 rupees, but they aren’t keeping their promise, I write regularly to them, first someone named Harjit told me that my cashback is approved and I will receive it on first April, but I haven’t received it yet, I wrote to them again and now they say I haven’t applied the promo code , they are cheating me.