Deductions & credits

As Community user supratt pointed out, you will have a "statement" included with your K-1 with one or more amounts related to your Section 199A/QBI income.

You'll need to put the information on that statement into the categories shown in the "We need some information about your 199A income or loss" and "Let's check for some uncommon adjustments" screens (see two screenshots later in this post). 

Enter the code Z when you enter the K-1 box 20 screen, but you don't need to enter an amount.  Continue on, and you'll find the two screens.  When you check the box next to a category, a place will open up to enter your amounts.  These screens (if applicable to the amount(s) on your statement) must be completed in order for your box 20 code Z information to be correctly input into TurboTax. 

To get back to the K-1 summary screen and find the Schedule K-1 to edit, click the "magnifying glass Search" icon on the top row, enter "k-1" in the search window and press return or enter, and then click on the "Jump to k-1" link to find the K-1 you need to edit.

Here are the "We need some information about your 199A income or loss" and "Let's check for some uncommon adjustments" screens where you enter the information from your box 20 code Z statement: