I can't install some apps; it gives an error "Device is rooted".

Yes, this device is rooted.

If you want to unroot the device, please follow this instruction:

1. Download Root Browser app from here.
2. Copy the APK file to a USB stick.
3. Insert the USB drive to the media player.
4. Open the USB from media player File Manager or File Browser and install the Root Browser app.
5. Go to the Application folder and open the Root Browser app.
6. Open the System Folder > Go to Xbin Folder.
7. Select the file called 'su'.
8. Press and hold the OK button.
9. Select 'Delete' option and delete the file 'su'.
10. Reboot the Media player (disconnect the power then plug it back in).

Your Smart Media Player will now be unrooted.