We have a 2018 Max Platinum and I did the same thing as you, I removed the second row middle seat. When I did research on this initially, I am pretty sure where I read you need to make sure the seat belt was not latched. You might want to re-set the seat, plug in the harness. turn the truck on, and see if you get an error, latch the belt if needed. See if you can get the error to go away. Once the error is gone, then remove the harness, and check for the error again. Hopefully that works for you.
On another note, I found a company that will make you a 2nd row custom carpeted floormat with the middle "isle". I also ordered a custom 3rd row mat. Total was almost $200 but so very worth it. Especially to have a 3rd row floor mat when Ford doesn't give that as an option.