I completely understand how challenging it can be to see that error, @cr8vrlj, especially when you know you haven't used that number on a visible invoice. I've gathered some information about Sales Order conversions and numbering that should help us fix the problem and get you back to invoicing smoothly.

QuickBooks Online (QBO) aims to protect your financial data by ensuring every transaction has a unique ID. When you convert an estimate to an invoice, the system immediately reserves the next available number in the sequence for an accurate audit trail. Sometimes, due to a momentary data interruption. The system successfully reserved the next number in the sequence for the conversion, but the invoice wasn't fully saved within QBO, prompting you to use the next or a different invoice number.

We can rectify this by using the Custom transaction numbers feature. It will help us temporarily skip that reserved number and direct the system to the next clean starting point.

Here's how:

1. Go to the Gear icon and select Account and settings.
2. Navigate to Sales, then the Sales form content section.
3. Turn Custom transaction numbers on.
4. Click Save, then tap Done.

Next, create a new invoice and manually enter a number that is safely past the last number you see (Example: if the last one was 1012, we'll enter 1022). It will tell QuickBooks that the new sequence starts after this number. Once done, save the temporary invoice and try converting your Sales Order again.

Once everything is working correctly, you can safely remove the temporary invoice by deleting or voiding it. After that, return to the Account and Settings menu and turn off the Custom transaction numbers feature to restore the default numbering system.

Let me know by leaving a comment below if you encounter any difficulties while following these steps or if the problem persists. We're here to support you and ensure your QuickBooks experience is as smooth as possible.