How to enter discount to reflect one item, on a invoice where are listed more sales items?

You are correct, usernatasha.

The Discount feature can only be applied to the whole transaction. Here's a link for reference about it: Add a discount to an invoice or sales receipt in QuickBooks Online.

As an alternative, we can manually create a discount item that we can use to indicate the discounted item. Here's how to do it:

Create a discount account

1. Click Accounting on the left menu and go to the Chart of Accounts tab.
2. Click the New button and select Income in the Account Type field.
3. Select Discounts/Refunds Given in the Detail Type field.
4. Enter the Name and click Save and Close.

Create a discount item

1. Click Sales on the left menu and go to the Products and Services tab.
2. Click the New button and select Non-inventory or Service.
3. Enter the Name and go to the Description section.
4. Select the discount account in the Income account field.
5. Click Save and Close.

We can now add it to the invoice as one of the line items. Here's how:

1. Open the invoice you want to add the discount item.
2. Add the discount item and enter the Description if necessary.
3. Enter the amount as negative.
4. Click Save and close.

Currently, that is the only workaround that I can think of. You can create as many discount items if necessary.

Keep us posted if you have additional queries regarding this matter. Have a great day!