I cant set up purchases taxes in QuickBooks Online

Hello, Angela888. 

Setting up taxes in Purchases, you need to make sure to set up the tax first. Here's how:

1. Click Taxes on the left pane.
2. Select Set up tax.
3. Set up the necessary details ,then select Next.
4. Follow the steps to finish and select Done. 

You'll see a Tax/GST column on your sales and purchases forms. 

In case you want to create a custom tax rate, please refer to this article: Create custom tax rate. 

Additionally, check this article on how to apply sales tax to transactions: Applying Tax to transactions and sales forms. 

As always, you can get back to me if you have any additional questions. I'm here for you. Have a great day and stay safe!