Hi there, @danish18.
Thanks for getting back and telling me about the options you're seeing on the Accounting Preferences. Allow me to help share further information.
The option to assign classes to accounts, items, and names is not available in the Premier version. You can only use this feature in QuickBooks Enterprise. Let me route you to this article site to compare the advanced features of Quickbooks Desktop Enterprise: https://enterprisesuite.intuit.com/resources/upgrade-to-enterprise/comparison-charts/.
If this is something that your company needs, you can consider upgrading to Enterprise. You can call their support number on the article provided whenever you're interested.
That said if you need anything else you can always reach out again in the Community. I'll be here to help.