The sales order feature is available for QuickBooks Online (QBO) Plus and Advance subscription, userinfo18.

If you're using a lower version of QBO plans such as Simple Start and Essentials this feature is unavailable. I recommend upgrading your QBO Online Plus or Advance.

If you are using these versions, let's begin by troubleshooting. First, add Intuit as a trusted site. Then, clear your browser's cached data and any cookies associated with Intuit.

Additionally, your computer's internet speed and web browser version can affect the software's performance. To ensure a smooth experience, make sure you are using the latest version of your browser and that you have a stable internet connection.

For a comprehensive guide on using Sales Orders in QuickBooks Online, including how they work with different subscription levels, check out this article: Create and manage sales orders in QuickBooks Online.

You may also check this article for your future reference about sending invoices: Send an invoice to a customer.

If you still have further queries, reach us here in the community support.