How to reverse a Purchase Transaction?

["Nick Prater" <..hidden..>]

I have made a mistake entering an invoice received from a supplier/vendor,
via Purchases->Add Transaction.

What is the best way to reverse this?

I can find the bad transaction via Purchases->Reports->Transactions. There
is a 'Delete' button, but that gives "ERROR: permission denied for
relation ap".

I'd appreciate any guidance before I make matters worse!

Many thanks and best wishes,

Nick