Hi Lucy! Thanks for checking in!
Invoices are legal documents and need to have a consecutive numbering system, so - as per HMRC’s rules - you can’t delete or edit an invoice once it has been issued. Instead, you will need to raise a credit note against the relevant invoice to write it off.
A credit note can be raised for the full amount of an incorrect invoice, and then another invoice issued to correct any mistakes.
I’m sure that’s not the answer you’re hoping for but I hope that clears up why the Edit button isn’t functioning!