Terrible experience.

I have been trying to cancel my contract. Due to health issues. They want me to give a doctor's letter as to why. Which I did, according to them the information given is not enough. I even cancel my contract Online, but they keep on messaging me that, I owe them. I didn't even go to the gym once, but I was already billed. I feel that I don't need to share my health problems indept, its not their business. Just cancel the contract.