Diagnosis
That's likely uniformity banding, a common panel artifact on IPS monitors like the U27P2. It's not a dead pixel line, but a subtle variance in backlight illumination or panel pressure.
How to Fix It
Unfortunately, there's no OSD setting to fix a physical panel characteristic. First, try gently massaging the area around the line on the screen (with the monitor off) using a microfiber cloth. Sometimes it's a slight pressure point from the bezel. If that does nothing, the only software-side mitigation is to avoid very low brightness. Crank the brightness up to 70-80. Uniformity issues are often most visible at low brightness levels (like 20-30).
If That Didn't Work
Run a pixel refresher. While the U27P2 doesn't have a manual OLED-style refresher, leaving the monitor powered off and unplugged for a few hours can allow the LCD panel components to reset, which has been known to minimize temporary image retention and sometimes improve uniformity.
Heads Up
All IPS panels have some degree of uniformity variance, often described as 'IPS glow' or 'backlight bleed' in the corners. A faint vertical band is within the realm of common manufacturing tolerance unless it's extremely pronounced or multi-colored. If it's truly distracting, you'd need to pursue a warranty exchange, but be aware the replacement panel might have a different uniformity quirk.