There might be different reasons for why you are unable to start a subscription, although we will try to help you with the most common reasons.
1. You click on a subscription which you are already subscribed to. If you already have a subscirpition on a channel, you won´t be able to start a new one, instead you get a message saying the page doesn´t exist. (it does exist, just not for you whom already have one :) )
2. Your card you are trying to pay with doesn´t have internet payment or subscriptions allowed. Please contact your bank to check.
3. The team/club/league has unpublished the subscription.