Q: Cannot unsubscribe

You've tried on both your phone and computer's iTunes ? If you can't turn the subscription off on either then try contacting iTunes Support (did you get a reply when you contacted them originally, if not has it been more than 48 hours since you contacted them ?) : https://www.apple.com/emea/support/itunes/contact.html

Is it possible you were not paying through iTunes (like, you paid it to Smule themselves, or another store than iTunes), or you paid using another Apple ID?

From what I understand, your account needs to be in good standing if you are looking to manage it.

they said me that I didn't pay for one invoice(this one) and ask me to update payment information

That implies that iTunes wasn't able to collect the money for the last week's subscription, and maybe why you aren't getting the option to turn the subscription off - does anything show as being owed on the account : Pay an unpaid balance in the iTunes Store - Apple Support ?