Transactions > Customers > Issue Customer Refund > Unexpected Error: Transaction was not complete.

Solution:
Use the standard form or customize the form and enable the Account form.

1. On the new refund page, check if the Account field is missing.
2. If it is, customize the form and check the Account box under the Screen Fields tab > Main subtab.
3. Save the form

The account field is the bank account used for the refund and this is mandatory.