The main supplier's item number, not your item number, is set up to print on item purchases

Items Overview > Other considerations when creating items > The primary supplier's item number is set up to print on item purchases

MYOB Accounting Plus item purchases are currently set up so your suppliers' item numbers appear on the purchases you print using Accounting Plus. If you use a different system of numbering your items than your supplier, this might seem confusing to you when you first print your purchases. If you wish to change the way your item purchases print-in other words, if you want to print your own item numbers on the purchases-you'll need to customise the item purchase form.

Click below for the step-by-step procedure:

To print your item numbers on an item purchase