Hi Emptyshed,
Welcome to the Community Forum!
You’re totally smashing it by customising your invoice templates for each client, all while handling a hectic full-time gig and not much backup, credit to you for that! Those errors usually show up when something like an account (or category) isn’t linked to an item or line in your invoice, or if you’ve missed adding a tax code for your item or category.
Since MYOB needs a tax code on every invoice and category, have a quick look at your category setup and make sure they all have a tax code in place. If you’re not registered for GST, the N-T (No Tax) code is your usual go-to, but just to be on the safe side, have a chat with your accountant to confirm the best tax code for your business.
Cheers,
Princess