Receive message can't assign new no invoice

Hi there, @accounts414.

I'm delighted to answer your first post. I'm here to help ensure you're able to assign a new number to the invoice.

We can check the Custom transaction numbers in the settings and make sure it's checked. Here's how: 

1. Click the Gear icon, then select Account and Settings.
2. Select Sales, then select anywhere within the Sales form content section.
3. Ensure that the box next to Custom transaction numbers has a checkmark. Otherwise, check the box next to it to enable the feature. 
4. Click Save and then Done.
5. Open an invoice or any sales form, then enter the custom number in the Invoice no. field

Check out this article for more insights: How to change the invoice number on the sales form.

For future reference, read through this article in case you want to learn about customising your invoices, quotes, and sales receipts in QuickBooks Online.

I'll be around if you have other invoices concerns or questions in your QBO account. I'd be happy to help.