In order to fix the issue, press Windows + Pause/Break key combination and open Device Manager. Expand the Universal Serial Bus controllers list, right-click on the driver and select Uninstall. Reboot your computer, connect the MP3 Player using the USB cable and press Scan for hardware changes icon from Device Manager. After that, launch Windows Media Player and try manually to add songs to the device.
If it doesn't work, navigate to Start > Control Panel > Uninstall a program and press Turn Windows features on or off button. Expand the Media Features list, un-check the Windows Media Player box, click OK and reboot your computer. After that, enable again Windows Media Player using Turn Windows features on or off tool and try again to transfer your files.
Alternatively, you can transfer the songs by opening the MP3 Player drive from My Computer/Computer and using the Copy/Paste feature.