The problem of the Huawei P9 is due to a configuration error that can happen in a random way: when we connect the smartphone to the computer via USB, the PC does not recognize the phone, preventing us from messing with it.
To solve it we will have to follow two steps: check that our PC has the latest version of HiSuite (the Huawei file manager) installed, and if not, update it from the application itself.
Once this is done, we must go to the P9 settings and in the "Advanced settings" option click on "Security". When we get this menu, we must make sure that the option "Allow HiSuite to use HDB" is activated. If not, we do it.