I couldn't select tax rate when i try to categories transactions. can you please advise how i fix that ?

Thanks for bringing this to our attention, Dimpy. I'll add some insights regarding your concern with sales tax rate below.

Before we begin, I'd like to ask a few questions about applying tax rate, are you trying to add a tax rate under products and services or select a tax rate for categorizing bank transactions? Your answer will be greatly appreciated as this will help us isolate your concern.
 

Meanwhile, we can perform some basic troubleshooting steps. We can use a private or incognito window to check for browser cache problems.

Here's how:
 

To begin, sign in to your QuickBooks Online (QBO) account in an incognito window. It will prevent the browser history from being saved.

Please refer to the following keyboard shortcuts based on your browser type:

• Google Chrome: Ctrl + Shift + N
• Mozilla Firefox: Ctrl + Shift + P
• Microsoft Edge: Ctrl + Shift + N
• Safari: Command + Shift + N
   

If operating in a private browsing session resolves the issue, you can then clear your browser's cache to facilitate a system reboot. This also applies to other supported and up-to-date browsers. Clearing the cache helps remove stored data causing issues.
 

Moreover, you can refer to this page to learn how to add sales tax rates in QBO: Apply sales tax rates to transactions and sales forms in QuickBooks Online.
 

Furthermore, you can also read this page to learn how to calculate sales tax in QBO: Set up and use automated sales tax in QuickBooks Online.

We’re keen to ensure your issue is addressed, Dimpy. Please know that you can revisit this thread if you have further questions with regard to your sales tax. We're always here to help.