After a few weeks, estimates and invoices suddently not allowing me to add taxes. How can I fix?

Hi mike-thebusiness,

Welcome to the Intuit Community! I'm glad to hear you're taking advantage of the sales tax center in QuickBooks Online to track every sales tax you collect and pay. At the moment, there isn't any reported issue with the tax drop-down on sales forms, and I want to make sure you can use this tool so that nothing is missing on your end.

A good starting point would be to ensure there are no bugs in the software due to a browser issue. To ensure your app or browser is running the program as designed, I recommend following the troubleshooting steps in this article: Clear cache and cookies to fix issues when using QuickBooks Online

Another good troubleshooting step to resolve these kinds of situations is to navigate in a private/incognito window or use a different browser compatible with QuickBooks Online. Here's an article with the list of supported browsers: System requirements for QuickBooks Online, Accountant, and QuickBooks Self-Employed

I hope this helps you get back on track! Let me know if there's anything else I can do to help.