I am new to qb online. They imported all my data from desktop. Each time I try and enter a sales receipt it is giving me a notice saying that number is already used ???

I'm here to help make sure you can enter the sales receipt's transaction numbers in QuickBooks Online smoothly to avoid duplicates, accounting-olme-.

Once you enter a number, QuickBooks will automatically increment it to avoid duplicates. As a workaround, you'll need to enable the Custom transaction numbers option from your Account and Settings so you can enter your desired digits.

1. Click the Gear icon, then select Account and Settings.
2. Select Sales from the left menu.
3. Go to the Sales form content section and toggle the Custom transaction numbers to turn on the automatic numbering.
4. Hit Save, then Done.

Check this article to learn more about setting your own numbering preference: How to change the invoice number on sales forms.

Get back to me if there's anything else you need by commenting below. I'm always right here to help ensure you're able to enter sales receipts with no issues.