Thanks for the photos, that is either a power supply fault or some bad leds as the most likely cause.
It is possible it is a panel defect as well, but not likely to be burn in.
This issue could be a manufacturing defect, but is also often caused by a power event (shock/surge, prolonged over/under voltage condition, etc).
This could also be the result of age related wear and tear; as a 2015 model tv, that type of failure wouldn’t be surprising for this brand. These are some of the lowest quality tvs on the market. They are built to meet a certain price point, not for quality or reliability. It is completely normal and expected to see them last just a year or two at best.
There is one step you can take here, both as a possible fix, as well as a diagnostic evaluation:
1) Unplug the power cord from the outlet
2) Wait at least 1 hour (60+ Minutes)
3) Press and hold the power button (on the set, not the remote) for 60 seconds
4) Keep holding power button while plugging back in, and for another 60 seconds after. When plugging back in, go directly to the wall outlet, bypassing any power strips or surge protectors.
If this reset process does not work, it would unfortunately confirm an internal hardware failure.
This is something that would require repairs to correct; if you're thinking about a do it yourself repair, you'll need to be comfortable and skilled with electronics repairs, which could include dismantling the set (removing the back panel of the tv), diagnostics testing with a multimeter, changing out boards as needed, and possibly desoldering/removing and soldering/replacing individual components.
If you’re considering repair at a local shop, you’ll find a typical repair to be around $300 for diagnostics, parts, and labor, assuming it is not the panel (which can't be fixed)
For a tv of this brand/age however, replacement will make more sense. You can get a brand new 4k LED tv with a fresh warranty, from a more reputable brand, in the $700 range.
I can help you with finding parts, finding a repair shop, or recommending replacement options, if desired.
My apologies for the news,