As a workaround, you can try "Ctrl+Alt+e" on your keyboard or select "+New" from the left Dashboard and select Estimate under Customers. If you want to add a new customer, select "+ Add New" from the Choose a Customer field. Select "+Details" if you need to fill out Customer Information.
Let's perform some troubleshooting steps so you access the Customers tab and create an estimate for your new customer.
To check if it's a browser data issue, let's try logging in to your QuickBooks Online account using a private browser. There are times that the browser stores a frequently-accessed data, thus websites, like QBO, to act weirdly.
To use a private browser, here's how:
Google Chrome: Ctrl + Shift + N
Internet Explorer: Ctrl + Shift + N
Mozilla Firefox: Ctrl + Shift + N
Safari: Command + Option + N
Once logged in, go back and try to access the Customers tab and create an estimate again to double-check.