[Huawei P9 ROMs, Kernels, Recoveries, & Other Devel] Tecalote:
Okay, I have seen your PM and your Build Number is L09C432B378.

I would do following:

A) Download Full Firmware b378

1) Update.zip: http://update.hicloud.com:8180/TDS/data/files/p3/s15/G753/g104/v74832/f2/full/update.zip

2) data.zip:

http://update.hicloud.com:8180/TDS/data/files/p3/s15/G753/g104/v74832/f2/full/hw/eu/update_data_full_hw_eu.zip

B) Download Huawei Update Extractor:

https://xdaforums.com/showthread.php?t=2433454

C) Unzip Update.zip on PC, you get some files but need only UPDATE.APP

D) With Huawei Update Extractor open Update.app and extract BOOT.IMG and RECOVERY.IMG (not recovery2.img)

E) Be sure you have enabled in Developer Options ADB Debugging, USB Debugging (and of course OEM Unlock) With Minimal ADB and Fastboot (in administrative Mode) flash only Boot.img first.

F) Be sure you use latest version this TWRP:

https://xdaforums.com/p9/development/twrp-t3565703

After flashing boot.img restart System without trying to Root, just to see if your external SD is available.

G) If external SD is not available, shut down your device and move out your SD. With Adapter connect SD Card to PC and check if the file system is okay - if not repair the file system. Maybe SD is corrupted.

(Try backup files, perhaps format it with ExFat)

After repair SD do it back into your device and start your phone.

H) If SD can recognized make Huawei Backup from all your data and apps.

If SD don't work, try another SD.

If SD works - I would restore whole system with next step.

I) If you have TWRP Backup from unmodified Stock Firmware b378 restore boot, data, system and vendor

If you don't have:

J) Make folder dload on your external SD and copy from update.zip only Update.app into this folder. If no SD Card works make the folder dload with update.app inside on internal memory. But better into external SD (don't know if it works on internal memory) - so you need a way, external SD can recognized.

K) Flash now Stock Recovery (you have extracted: RECOVERY.IMG) - cause updates are only possible with Stock Recovery.

L) Disconnect USB from device and boot to Stock Recovery with Vol+ and Power Button.

M) In Stock Recovery wipe first Cache Partition and after make factory reset.

N) Boot into system and make just a short Setup, but register your Huawei ID. Look, if you get an OTA to b386. (SIM must be inside, Akku must have min 80%)

If you get OTA, update your Firmware to b386 (its much easier to Root, has better battery life)

O) If you don't receive OTA - restore whole Firmware b378 first. You have already update.app in dload.

With Vol+ and Vol- (press down in the middle and holding) and Power Button restart Phone. Keep holding Vol+and Vol- down - only Power Button release in the moment Huawei Logo is on Screen.

EMUI Software Installation will restore b378.

P) After booting to system - delete update.app in dload folder.

Now unzipp update_data_full_hw_eu.zip on PC.

After copy all files you get (Meta-Inf, data and update.app) into dload folder.

After restart Phone with same Buttons (Vol+ Vol - and Power Button) and let EMUI install the necessary files.

Q) After booting to system, boot once again to Stock Recovery (Vol+ only and Power) and wipe Cache and make Factory Reset.

R) Make complete Setup and you should receive after the OTA to b386. Install this.

If everything is working then I tell you how to Root.