Hello there evakozo,
Thanks for posting here on the Apple Support Communities! This is a place where other Apple users can submit questions or respond to others to help lend a hand. Let's take a closer look as we do our best to provide some additional insight.
We understand you're having some issues with iTunes when you go to manage your subscriptions. To get started, if you have another compatible device, you can try to cancel the same subscription using the other supported methods here: How to cancel a subscription from Apple
Otherwise, try restarting your computer or logging into to iTunes on a new or different user account on the same computer. If you're still not able to cancel the subscription or don't have access to any other supported devices, contact Apple Support for additional help.
We hope this helps get you pointed in the right direction.
Take care.