Move apps manually to system
Installing apps on the system partition is not possible without root. Rooting can also not be enough if you are running a device that has a newer android version, where selinux is enabled and enforcing. 

You need to make sure you do the following before being able to install apps to system:


Bootloader unlock (if needed)
Root
Mount /system as RW
Copy apk file to /system/app (or priv-app) (depending on the OS version)
Remount /system as RO or reboot


You might also be able to do it through recovery, but at that point you need to run a custom one, as TWRP, the only difference being that you don't need root at this point.