I’ve applied the voucher but now I’m getting charged

If you cancel your subscription after the voucher has been successfully applied, your subscription, and the voucher, will be voided. You will need to contact Kayo support to either get a new voucher or have credits added to your account for the remaining period of the voucher.

https://help.kayosports.com.au/s/article/12-months-of-Kayo-Basic-with-AFL-Digital-Membership#cancelled