You may encounter a scenario where customer does not have an ABN and you are happy to on board the customer without an ABN. You can disable this option in Customer Settings bye following the steps:
You can toggle OFF the setting that requires the customer to input the ABN by following the steps below:
Go to Customers -> Search for Customer -> More Actions (...) -> Customer Settings -> Toggle OFF the "ABN Required" setting.
If you have already sent the on boarding email to the customer, ask them to refresh the page. This will remove the ABN field and and they should be good to go. Please see screenshot below: