Can you please elaborate with some more details. Lets start with what OS is installed on your iMAC and which version of the CC app are you trying to install? Did you check the minimum system requirements for the CC app and make sure that it matches the capabilities of your iMAC?
The following link should help you verify these details
If you just bought the subscription, go to https://account.adobe.com/plans and cancel your plan. It will be free of cost, and you will be refunded. If your subscription is older than 14 days, you can cancel, but there will be no refund, and you may need to pay an early termination fee of half of the remaining due.