Hello there, @Judith26.

In QuickBooks Online (QBO), we can turn off the Custom Transaction Numbers option if you want to use the number sequence. 

Here's how:

1. Go to the Gear icon.
2. Under Your Company, select Account and settings.
3. Click the Sales tab.
4. Select the edit pencil icon, then turn off the Custom transaction numbers.
5. Hit Save and then Done.

For future reference, you can visit the How to change the invoice number on the sales forms article. It also provides information about assigning numbers to your sales forms automatically.

Reach out to me whenever you have additional questions or concerns. Please know the Community has your back, and I'm here ready to help you. Have a good one.