De Lijn ticket machines at Zaventem airport

I don't know them , but you can buy an e ticket with app or you can buy an sms ticket at almost the same price as at a ticket machine.
Buy these tickets before you board the bus.

https://www.delijn.be/en/vervoerbewijzen/verkooppunten/ticketautomaat-aa....
Search 'Zaventem'. The results are in English and include the airport.