Did you follow the steps as below (search: "your model nr" odin):
Download Odin3_v3.12.5.zip and unzip it.
Download and install Samsung USB Drivers on your computer if you don’t have installed them already.
Go to Settings> Developer options and enable USB debugging.
Backup your apps and data as a precaution.
Unzip the firmware zip file you downloaded from above. You will get a file with .tar.md5 extension inside the extracted folder.
Boot your Galaxy J7 into the Download or Odin mode. To do so, turn off your phone and press and hold the Volume Down + Home + Power keys at the same time for 2-3 seconds to boot your phone into the Download mode. Release the keys as soon as you see a blue screen with exclamation sign and press the Volume Up key.
When your phone gets into the Download mode, open the Odin folder double-click the .exe file to launch it.
Connect your phone to your computer using the USB cable that came with your phone.
When your device is detected by Odin, the ID:COM port will be highlighted (see below).
Source: droidviews com