I tried to perform the steps here on my end and was able to create and estimate. A browser-related issue is a possible reason why you get this unusual behavior.
We can perform a few browser troubleshooting steps to fix this. Here's how:
Let's use a private browser to access QuickBooks, and create an estimate through the customer's page.
If you were able to do so, we can clear your regular browser's cache to remove piled-up cache that can cause other options in QuickBooks not to function well.
You can also use other browsers such as Google Chrome, Internet Explorer or Mozilla Firefox.
If you still can't open the estimate window, I suggest contacting our phone support agents so they can further investigate. From the Help ( ? ) icon, click Contact us. Enter your concern and then proceed to Continue. You can Start a message with us or Get a callback.
In the meantime, you can use the Plus (create) icon at the upper-right corner when creating an Estimate.
I'm always here if you need further help. Have a great day!