How to make entries for purchase (COGS) against the sales in quickbooks online plus

Thanks for posting in the Community page, @shanbala99.
 

A way to track the Cost of Goods (COGS) is to make sure that there are income and expense details in your inventory items.
 

Also, you can manually track the value of your inventory assets in QuickBooks Online. To start, you can create an account to track your inventory value. Here's how:
 

1. Select the Gear icon, then Chart of accounts.
2. Click New.
3. Select Current assets from the Account Type drop-down.
4. Select Other current assets from the Detail Type drop-down.
5. Hit Save and Close.

Second Create an account to track your cost of goods sold.
 

1. Select the Gear icon at the top, then Chart of Accounts.
2. Click New.
3. Select Cost of Goods Sold from the Account Type drop-down.
4. Select the closest type of Cost of Goods Sold that matches your situation from the Detail Type drop-down.
5. Hit Save and Close.

When you purchase inventory using Checks, Expenses or Bills, use the asset account you created to track its value in the Account field. This "transfers" the money into the asset account, increasing the value of your inventory.

If you're using QuickBooks Online Plus, you can use the built-in inventory feature instead of tracking inventory manually.

You can refer to this article for further details: Manually track inventory.

Please let me know if you have any other concerns  in QuickBooks Online, I'll be right here to help. Have a good day ahead!