[Huawei P9 Guides, News, & Discussion] zgfg:
Sorry, I don't know anything about b136. I don't understand, you were two days ago on b210 and asking how to update to Nougat but then you went even further back, to one of the first and oldest MM firmweares b136

For rolling back to MM, b182 was good firmware - latest, stable MM (EMUI 4) firmware, and if rolled back from Nougat to b182, there was no problem with OTA to go back to Nougat (e.g., I did it several times and guided few others).

But what is going on with that old b161 and b136, why your attempts to upgrade to Nougat fail, no idea

Usually, if you followed some guides about b161 and b136, then in those threads you should look for guidance and possible links about upgrading.

Nowadays on Firmware Finder there ate no such old firmwares anymore and it is hard to find them.

People who had P9 and had their own resources on Mega, G Drives, etc, moved on and you will only find dead links. I had firmwares I downloaded from elsewhere and used them successfully but when I sold my P9 I also cleaned up to make room for the other phones (moved to Mate 9 and now Xiaomi Mi 9T)

You can try with Service repair packages (but they have only Nougat, b387, b388 and b389):

https://androidhost.ru/search.html

Put EVA-L19c432 to the Search box. Download, unzip, untar (packages might be archived differently) but each eventually contains two folders: Docs and Software (names can vary) . In the Docs folder you will find Huawei original (in English, for the worldwide Huawei Service shops) Installation guide (which zip files from Software folder to use, how to rename or uncompress them if needed, etc) how to do (forced or unforced) DLOAD installation. Guides also describe what should be the minimal version applicable to start from (I don't know if that old b136 is supported, you will need to look).

You may try even if b136 is not mentioned there (people e.g. unbricked Huawei phones with Service repair packages, they are really worth to try)

There is also another site with various firmwares (go to Home and browse from there, they have tons of things but not sorted friendly, you could find for P9 in various folders, under P9 or under EVA, etc):

https://support.halabtech.com/index.php?a=downloads&b=folder&id=5823

https://support.halabtech.com/index.php?a=downloads&b=folder&id=35186

Once I found there Service repair package for Mate 9, EMUI 9.1 (i.e., also suitable for the simple DLOAD installation) - it was not possible to find it anywhere else (EMUI 9 firmwares on Firmware Finder are not applicable for DLOAD and the first link I gave you above rarely has any EMUI 9.1 or newer package), but earlier I downloaded few other firmwares and found that they were nothing else but repacked packages from Firmware Finder; or another one I downloaded required password to uncompress and when I asked them by mailthey answered they don't know and don't care about, they only provide the upload/dowmload service)

Downloading from that side is also perky without a paid subscription, there are limits on downloads per user, per all users per day, in GBs, per particular file, on concurrent downloads, etc) - good luck with that site if you go for