Seven things have to appear on a valid Australian tax invoice, and an eighth once the sale reaches $1,000. Work down the list against an invoice you have actually issued.
7 items are missing
A missing ABN is not a formatting problem. Where a business customer receives an invoice without one, they may be required to withhold 47% of the payment and send it to the ATO. You would be paid less than half, and getting it back means waiting until you lodge. Every other item on the list is compliance; this one is cash flow.
General information only, current at the time of writing. It is not tax advice and does not account for your circumstances. For anything that matters, check with your accountant or the ATO.
BookAndGo builds the invoice from the appointment, so the eight items are simply there. Your ABN and business details come from your settings, the GST is worked out for you, and the numbering runs in sequence without you tracking it.