@Robin851 You may already have tried this, but:
Leave the reservation and refund pages and go to your account (click on your picture in the upper right). Choose "payments and payouts."
Choose "payouts," the second option along the top.
Choose "add a payout method."
Put in your bank account details. You might have to wait until it's verified.
Only after your bank account has been verified can you issue the refund. Assure the guest that you will get her refund to her as soon as everything has been verified.
Once verified, go back to the reservation and choose "Send money." Then issue the refund from there.