I cannot purchase any tickets
It is unlikely that you will not be able to find tickets for any zone at all. But if this happens, this is because there are none available at that time (we advise you to check back later) or because sales of tickets for this match are closed (the website will indicate whether there are plans for a new sales period to open) or maybe there is a technical problem with the website (a message should appear explaining this and directing you to another valid sales channel).