With error messages in QuickBooks Online, I'd like to start with some troubleshooting to see if it's a browser-related cause. Let's begin by clearing the cache of the web browser you use for your account. These files can become outdated, causing issues like the one with not being able to create an Estimate. Here's a link to help you with the process: Clear Web Browser History.
Let me know if this does the trick! I'll be here if you need me. Have a beautiful day.