To see if it is a temporary glitch, try a "forced" restart of your device using these instructions: Put it on charge for at least half an hour, then, keeping it plugged in, use these gestures:
Press and quickly release the volume up button, press and quickly release the volume down button, then press and hold the side (power) button. Only when the Apple logo appears (it might take 10-20 seconds), release that button (ignore all other screens that might appear, the one with the "slider" to turn the device off included).
If the issue persists, and if that line appears in ALL pages, yes, it could be a hardware (screen? other internal component?) failure but we, just users like you, cannot diagnose that issue remotely, sorry.
Since your iPhone is still covered by warranty, I suggest that you contact Apple Support (they have tools to check your device remotely) using this link:--> Contact Apple for support and service or, even better, make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> https://locate.apple.com/choose-country-region . After checking it out, they'll give you all the available options.
Regards
Giulio
(*) You can do it: