There are two possible reasons why you're encountering the error message. I'd be glad to lay down the details and the solutions so you can delete the sales transaction.
First, this is likely a simple connection issue that can be resolved by refreshing the page. You'll want to press the F5 key on your keyboard while on the sales receipt screen. Then, delete it again.
You can also log out and back in by clicking the gear icon, then selecting Switch company. Then, click the Logout link from the Company selection screen.
Another reason is that this is a case of a cache-related problem. Let's use a incognito or a private window first to isolate this. The shortcut keys:
Google Chrome: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + P
Safari (newer versions): Command + Option + P
Next, go to your QuickBooks Online company, then delete the sales transaction. When you're able to do so, go back to the regular browser and clear the cache. Close it afterwards to complete the cache clearing process.
If you're using the QuickBooks Online Windows or Mac app, you'll want to click the Help menu, then select Reset App Data.
After dealing with the transaction, I'm sure you have other things to do in QuickBooks Online. You're more than welcome to browse our articles here. They have guides on how to run reports, create other transactions, or manage your VAT or other liabilities.
Do you have other questions for QuickBooks Online? Create a new thread or post your reply here and I'll help you out again. Happy new year!