How can I cancel a pending purchase?

I forgot to cancel a subscription after the free trial. I was charge for the subscription but I think because the app is already offloaded in my phone it didn’t really charge completely. I cancelled the subscription and I was supposed to report a problem so that I could get a refund because it charged me in my visa. But I can’t because the purchase is still pending. It’s been days and I still can’t get my refund. Please Tell me what I could do. Thank you!