I cannot update my J7 Prime and the device status is "Custom"

[Android Central Question]

My Samsung Galaxy J7 Prime was updated to Nougat official last year. Few days back, I tried to factory restore my device and after the restart I got blank screen not moving forward. I took it to a technician and he got to fixed in Marshmellow 6.0.1. Now, The device status if Custom and I cannot update the Android version as It is showing the message: "The Operating system on your device has been modified in an unauthorized way. Try downloading software updates using Smart Switch on your PC or visit a customer service center." I tried to connect it through Smart Switch but it notified that It is unsupported device. Also I tried wiping system cache and removing SD card but nothing worked.

Please help me out as It is giving me headache