I cannot see the price rules option in account and settings. How to activate it ?

The feature is only available in QBO US version. For other QBO regions, utilize an inventory management app to do so. 

Hello there, @nohotech1205, 

Currently, the price rules feature is in beta testing and available in the US version of QuickBooks Online (QBO). That's why you're unable to see it on the Account and Settings page.

I've got two workarounds you can perform to complete your task. First, I'd suggest manually entering the appropriate price in the Rate column when creating an invoice. View the screenshot below for your reference. 

Second, you can look for an inventory management app that's available in your region and can integrate with QBO.

For the time being, I recommend visiting our Resource Centre so you'll stay updated on our product innovations. 

In case you need to turn on other features on the Account and Settings page, check out this link for more details: Advanced Tab. 

I'll gladly help if there's anything else you need. Keep safe always.