Vertical lines are a common problem faced by DIY repairers after replacing the screen on an iPhone. Most of the time this isn't a defective screen and can be resolved with a little troubleshooting.
If the issue persists after trouble-shooting these steps you may have a defective part.
Check the connections between the cables extending from the display to the logic board. Dirt, dust or debris can be cleaned with a brush and isopropyl alcohol. Check and clean as necessary then reconnect to the motherboard.
ESD is a common culprit behind lines on the display. Too much was likely produced by the part itself or accidentally by the repairer on installation.
Use a ground to discharge any built up static charge before continuing the repair. Wear an anti-static wrist strap during repair to prevent this from happening.
The display has an IC chip that could be damaged. Check particularly on the upper left and right corners. Any signs of damage indicates the part is defective and needs to be replaced with a new one. tl:dr If this is damaged the part must be replaced.
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