I can't buy BOOM app subscribition

If it's a debit card then that might be why, not all seem to still be accepted by iTunes, and they might no longer be accepted at all in some countries. Do you have another card that you could try (preferably a credit card) ?

For a card to have a chance of being accepted it needs to be registered to the same name and address (including format and spacing etc) that you have on your iTunes account, and have been issued by a bank in the country where you are (and therefore the country which is on your iTunes account).

Apple ID payment information - If your payment method is declined, or you have a billing issue with a previous purchase in the iTunes Store

If necessary, contact iTunes Support. Either use the form or scroll to the bottom of the page in the 2nd link. Follow through the screens to get a chat or telephone option.

iTunes Support

iTunes Support – world wide