I am unable to import custom fields in a sales invoice. How to porperly do it?

I've got you covered, jatinjain01050-g.

When you're importing an invoice from a CSV file, you need to map the columns and make sure it matches with what you have in QuickBooks Online. 

If you haven't added the custom fields in QuickBooks, you can follow these steps:

1. Go to the Gear > Account and settings.
2. On the left panel, click Sales.
3. In the Sales form content section, put a check mark on the Custom fields and add the name of the column.
4. Click Save and then Done.

Please see this sample screenshot:

Here's an article for reference: How to add custom fields to invoices.

After turning on the option for the Custom fields, let's re-import the invoices.

Here's how:

1. Go back to the Gear > Import.
2. Click the Invoice tile.
3. Select the file from the Upload your CSV file drop-down menu.
4. Press Next. 
5. Follow the onscreen steps to map your columns.
6. Hit Next until finish.

You can use this article as a guide: Common questions about importing data to QuickBooks Online.

Once you have all the invoices imported, you might want to customise invoices, estimates, and sales receipts in QuickBooks Online. 

Otherwise, Import your custom form styles for invoices or estimates.

If you have other questions regarding QuickBooks Online, I'm here to help. Just reply here and I'll get back to you. Take care and stay safe.