Thanks for the details, Scott32.
First, the reason why you're getting an error upon importing is your Sales tax. In QuickBooks Online, importing Sales tax isn't possible.
What you're going to do is modify your CSV file and then remove the Sales tax column. Then, import your invoices again. Once the invoices were imported, you can edit it to add the Sales tax.
Next, you'll have to turn on the custom field you've created. This way, it helps you align the headers of your invoices inside QuickBooks.
Here's how to create a custom field:
To turn on this field:
Lastly, you're right about the Custom Data Field. It is where you can create the fields when importing your invoices.
This steps will help you continue your work inside QuickBooks. Have a great day!