Alright, finally got it to work successfully and placed order. Seems like lowes.com is having issues with me selecting local store pickup (kept timing out at final checkout page, -104 error), so ended up having it delivered (luckily still free).
In regards to coupon code, you essentially need to find a store where code has not been used for that zip code. If it fails, code's already been used for that specific zip code. Try finding a store in less populated areas with higher chance code hasn't been used yet. Good luck!