How can I turn "Tax on purchase items" on?

I can share some information about editing the tax rate you've created, bkim. 

If you haven't initially included the Purchase rate when setting up or adding a Tax rate in QuickBooks Online (QBO), the option to edit and update that information is no longer available. In this case, consider making the tax rate inactive or turning it off, then recreate the tax rate. When recreating it, ensure to provide all required details such as the Purchase rate. 

To inactivate the tax rate you've created, click the Make Inactive button beside the Edit icon showing in your screenshot. Then, follow the prompts to save the changes. 

Once done, recreate the tax rate:

1. 1. From your Sales Tax Center, click the Add tax button. 
   2. Select what type of tax do you want to add. 
   3. Provide all the necessary information. Make sure to add all the information needed including the Purchase rate.
   4. Click on Save once you're done. 

For more information on how to turn on tax tracking, set up tax agency details, and create tax rates, take a look at this article: How do I set up sales GST/VAT rates and use them on forms? (International QBO). It also guides deactivating tax rates and creating group tax rates to ensure compliance with tax regulations.

Additionally, you can refer to this article to learn more about recording, adjusting, and deleting sales tax payments in the tax center: Manage sales tax payments in QuickBooks Online.

Please let me know if you need further help in managing your tax rates in QBO. The Community is always here to assist. Have a wonderful day and stay safe!