Unable to install .apk from /system/priv-app I moved my apk file to /system/priv-app just as you adviced me in this thread. When I'm trying to install it through clicking this .apk, Android said me that "The application has not been installed". I tried to use adb install myapp.apk for this purpose but again ADB  gave me an error: Failure [INSTALL_FAILED_POLICY_REJECTED_PERMISSION] and I suppose that app requires permission which can be granted only by root. So the only way I see now is to run adb shell later su next cd /system/priv-app and install [-cdDsp] [-o USER] [-g GRP] [-m MODE] [-t DIR] [SOURCE]... DEST and that's a point because I totally don't understand this legend: 

    Copy files and set attributes
    -c      Just copy (default)
    -d      Create directories
    -D      Create leading target directories
    -s      Strip symbol table
    -p      Preserve date
    -o USER Set ownership
    -g GRP  Set group ownership
    -m MODE Set permissions
    -t DIR  Install to DIR`


How to set attributes to install this apk as system app?
I think you misunderstand the whole process. You are not supposed to use install command or the likes of it anywhere in the process I mentioned. You simply:


mount the system partition in rw mode
copy-paste the apk into the location, 
set the appropriate permissions using busybox's chmod and chgrp or through inbuilt tools:

adb shell su -c chmod 644 APK    # replace APK with your apk's absolute location, such as /system/priv-app/xyz.apk
adb shell su -c chown root:root APK   # replace APK as said above

do a reboot. 


Follow those instructions religiously. You can altogether avoid the commands using a file manager app such as Solid File Explorer, ES File Explorer, etc. 

During the following boot the Android would detect the apk and install it automatically.