You sure you didn't just plug in the cable from the logic board to charging port the wrong way?
I did this on a Google Pixel XL and it caused the phone to short circuit and not charge or turn on. Luckily just correcting the cable being plugged in reversed fixed it.
Not sure if it will work on the Huawei P9 but I always make sure I never mix it up. I have done screen replacements on Huawei P9 before.