There are several possible reasons:
The payment has already been refunded. This will be indicated in the initial payment details.
The payment status is not 'Completed' or 'Partial'.
You do not have the access rights to perform this type of operation: only Administrators, Developers, and Operators can refund a payment. If you do not have this role, contact someone with the access rights to finalise the refund, or ask the administrator to change your role in the 'Organisation' > 'Team' tab.