If you can't edit a transaction, it's usually because it's already synced. Some fields are also always non-editable, since they come directly from your card network.
Why a transaction can't be edited
Once a transaction is synced to QuickBooks Online or Xero, it's permanently locked in Melio. You can still view it, but you can't edit or delete it. Any corrections need to be made directly in your accounting software — there's no way to un-sync a transaction.
Fields that are always non-editable
Regardless of a transaction's sync status, the following fields can't be edited, since they come directly from your card network: amount, cardholder, card number, merchant, and date posted.
That's it! You now know why a transaction might be locked, and which details always come directly from your card network.