If you created offer with percentage discount and want to target free product, you will need to modify the discount's settings.

1. Go to Discounts in your store admin and locate the correct discount with the type "Buy 1 Get 1 item at X % off". Click on the discount's name to display detailed settings.

2. Switch discount code type from Buy X get Y to Percentage. Next, input the percentage discount value (the same as in the upsell offer). Lastly, in the APPLIES TO section, select Specific products and find the free product that you are targeting. Once done, do not forget to save the changes.