Using unique invoice numbers in QuickBooks Online (QBO) ensures transaction tracking, prevents duplications, and keeps your records orderly. Here, I'll walk you through the process of modifying the invoice number, peakind.

One possible reason you're unable to change the invoice number is that the customer transaction number feature is currently disabled in your system. This setting can restrict modifications to the invoice number. To enable the option to change the invoice number, ensure that the customer transaction number is activated. Here's how you can do it:

1. Go to the Gear icon and under Your Company click Account and settings.
    
   
    
   

    

2. Navigate to the Sales tab.
    
   
    
   
   
   
3. In the Sales form content section, tap the pencil icon and tick the Customer transaction numbers button to turn it on and Save.
   
   

    

4. Hit  Done.
   
   

Alternatively, if the feature is enabled but you still cannot make any changes, you can activate the Invoice number by accessing the Customization section of your invoice.

Here's how:

1. Hover over to the Sales menu, then Invoices.
2. Locate the invoice you want to modify and click on Edit.
3. In the upper right corner, click on Manage, then select Customization.
   
   

    

4. Tick the Invoice number button to enable.
5. Finally, hit Save.

I've found some helpful resources to streamline the process of recording and receiving your customer payments and personalizing your sales forms: 

• Customize invoices, estimates, and sales receipts in QuickBooks Online
• Record invoice payments in QuickBooks Online

 If you need further assistance with managing your invoices or any other QuickBooks features, don't hesitate to revisit this page. We're always here to help you streamline your financial processes and boost your business efficiency. Have a good one!