Why can't I edit an issued invoice?
An issued invoice is a tax document with its own number, and it is kept exactly as issued. Correcting it with a credit note keeps a clear record of what happened, for you, your customer and the tax office.
What about the email it sent?
If the wrong customer already received it, tell them it was a mistake. The credit note cancels the amount in your books. See Create a credit note for an invoice.
How do I avoid it next time?
Check the customer's name on the preview before you press Issue invoice.
Explanation written from the Deline app's own screens and texts (build eba146497 (Deline cf723fe1f)); it has no steps to check.