I'd be happy to assist you with deleting the unnecessary estimate that you made in QuickBooks Online.
To start with, I want to ask if you receive an error message while trying to do this. Any further details are much appreciated.
However, here's how you can delete an estimate in the system:
In the navigation bar on the left, go to Sales, then open your All Sales tab.
Locate and click on the one that needs to be removed. Use the Filter button and/or other sorting options if necessary.
If you're unable to do this, I recommend accessing your account using a private window and delete the estimate from there. This feature doesn't use the existing cache files, which makes it a good place to test if there's an issue with the browser.
Press the Ctrl + Shift + N (Google Chrome) keys on your keyboard.
Press the Ctrl + Shift + P (Firefox or Internet Explorer) keys on your keyboard.
Press Control + Option + P (Safari) keys on your keyboard.
If it was deleted successfully then we can clear your regular browser's cache data storage. At times, the stored data can cause some odd behaviors and simply needs to be cleared from the browser.