What you need to do
Additional reason is optional text. Where your country requires a statutory code, Corrective reason is required as well. That list is in Rules for your country.
Lines to credit shows the quantity the credit note will use. Change Credit qty only when you are returning whole units at the original unit price. A custom Credit amount is the payable amount to credit. The credit note shows quantity 1 at that amount, unless it divides evenly by the original unit price.
Prefill from original (negated) fills the lines as the opposite of the invoice. Leave those amounts negative. Saving a credit note positive is refused.
Click Create corrective draft. Then issue and send that draft the same way as an invoice, with Issue & send when it has to be registered. Until it is issued, it is a draft and the original is not yet corrected. Previous documents on the invoice shows the link, and whether the original is Partially corrected or Fully corrected.
