That is the error I get. I don't know why. I am connected. Well at this point I don't know if it refers to another kind of "offline" stuff. I didn't find any answers to this specific error
09/19 02:49:11: Launching 'app' on samsung SM-J710MN. Installation did not succeed. The application could not be installed. Installation failed due to: 'device offline'
There are two possible reasons for this behaviour. Either your Samsung device is faulty or this is an adb issue. To confirm, enter this command into your terminal while your Samsung device is connected:
adb devices
If the result returns the name of your Samsung device, then there's probably a problem with the Samsung device.
However, if your terminal does not recognize this command, it means you have to install Samsung adb driver on your computer. This way, your computer would easily recognize your Samsung device. For Windows, there are a ton of links out there where you can download Samsung adb driver; I have not verified any of them, so you might have to do the search by yourself. Nevertheless, I hope you understand the problem now.
I hope this helps.
Merry coding!