Download Huawei P9 Plus VIE-L29C185B181 official firmware from this post and follow instructions after the download options to learn how to flash the firmware.
This stock is specifically meant for VIE-L29C185Bxxx variants. Flashing on a different variant might cause a brick, if flashing is successful.
Download Options
File Name: P9 Plus Firmware (VIE-L29 Android6.0 EMUI4.1.1 C185B181 MiddleEast&Africa).zip
Size: 1.68 GB
Android version: 6.0
Firmware – Download
How to flash
Once you have downloaded the firmware from above link, click here to learn how to flash.