Mettle doesn't accept cheques, meaning that if you're due a refund from HMRC you'll need to let them know to pay this refund to you via bank transfer.
If HMRC have sent you a cheque you can contact them and ask to have the refund sent as a bank transfer. You can find out more on how to do this on the Gov.uk website here.
You must also return any out of date or unused cheques to HMRC.