If you're sure that nothing (cover, screen protector, dirt, scratch) is interfering with your touch screen in that area, AND If you already tried (to no avail) a "forced" restart of your iPhone, it might be a hardware (screen? other internal component?) failure but we, just users like you, cannot diagnose that issue remotely, sorry.
I suggest that you make an appointment (*) and have your device inspected at an Apple Store or AASP (Apple Authorized Service Provider) among this list:--> Apple – Location Selector. After checking it out, they'll give you all the available options.
Regards
Giulio
(*) You can do it: