See where to get your conversion label to plug into your AdWords Add to Cart Conversion Tag
Step 1: Go to AdWords > Tools > Conversions
Step 2: Create New Conversion
Once inside of conversions then click "Create" new conversion:
Step 3: Complete Conversion Details
These settings will be unique to your requirements. For example do you want to use last-click or a position based attribution model.
Once you save and continue then continue to get your conversion label when you implement via GTM:
Copy this conversion label to input into GTM.
Step 4: Paste Conversion Label into Add to Cart GTM Tag
Head back over to GTM and paste this value into your GTM tag as shown below.